diff --git a/plugins/calendar/calendar.php b/plugins/calendar/calendar.php
index 2c198f14..ba2f21e4 100644
--- a/plugins/calendar/calendar.php
+++ b/plugins/calendar/calendar.php
@@ -23,6 +23,7 @@
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
 
+#[AllowDynamicProperties]
 class calendar extends rcube_plugin
 {
     const FREEBUSY_UNKNOWN   = 0;
@@ -42,6 +43,7 @@ class calendar extends rcube_plugin
     public $timezone;
     public $timezone_offset;
     public $gmt_offset;
+    public $dst_active;
     public $ui;
 
     public $defaults = [
@@ -1049,8 +1051,9 @@ $("#rcmfd_new_category").keypress(function(event) {
             $old = $this->driver->get_event($event);
 
             // load main event if savemode is 'all' or if deleting 'future' events
-            if (($event['_savemode'] == 'all' || ($event['_savemode'] == 'future' && $action == 'remove' && empty($event['_decline'])))
-                && !empty($old['recurrence_id'])
+            if (!empty($old['recurrence_id'])
+                && !empty($event['_savemode'])
+                && ($event['_savemode'] == 'all' || ($event['_savemode'] == 'future' && $action == 'remove' && empty($event['_decline'])))
             ) {
                 $old['id'] = $old['recurrence_id'];
                 $old = $this->driver->get_event($old);
@@ -1447,7 +1450,7 @@ $("#rcmfd_new_category").keypress(function(event) {
         // $success is a new event ID
         if ($success !== true) {
             // send update notification on the main event
-            if ($event['_savemode'] == 'future' && !empty($event['_notify'])
+            if (!empty($event['_savemode']) && $event['_savemode'] == 'future' && !empty($event['_notify'])
                 && !empty($old['attendees']) && !empty($old['recurrence_id'])
             ) {
                 $master = $this->driver->get_event(['id' => $old['recurrence_id'], 'calendar' => $old['calendar']], 0, true);
@@ -1462,7 +1465,7 @@ $("#rcmfd_new_category").keypress(function(event) {
             }
 
             // delete old reference if saved as new
-            if ($event['_savemode'] == 'future' || $event['_savemode'] == 'new') {
+            if (!empty($event['_savemode']) && ($event['_savemode'] == 'future' || $event['_savemode'] == 'new')) {
                 $old = null;
             }
 
@@ -1472,7 +1475,7 @@ $("#rcmfd_new_category").keypress(function(event) {
 
         // send out notifications
         if (!empty($event['_notify']) && (!empty($event['attendees']) || !empty($old['attendees']))) {
-            $_savemode = $event['_savemode'];
+            $_savemode = $event['_savemode'] ?? null;
 
             // send notification for the main event when savemode is 'all'
             if ($action != 'remove' && $_savemode == 'all'
@@ -2131,7 +2134,7 @@ $("#rcmfd_new_category").keypress(function(event) {
         }
 
         // mapping url => vurl, allday => allDay because of the fullcalendar client script
-        $event['vurl']   = $event['url'];
+        $event['vurl']   = $event['url'] ?? null;
         $event['allDay'] = !empty($event['allday']);
         unset($event['url']);
         unset($event['allday']);
@@ -2153,9 +2156,9 @@ $("#rcmfd_new_category").keypress(function(event) {
             // 'changed' might be empty for event recurrences (Bug #2185)
             'changed' => !empty($event['changed']) ? $this->lib->adjust_timezone($event['changed'])->format('c') : null,
             'created' => !empty($event['created']) ? $this->lib->adjust_timezone($event['created'])->format('c') : null,
-            'title'       => strval($event['title']),
-            'description' => strval($event['description']),
-            'location'    => strval($event['location']),
+            'title'       => strval($event['title'] ?? null),
+            'description' => strval($event['description'] ?? null),
+            'location'    => strval($event['location'] ?? null),
         ] + $event;
     }
 
@@ -2460,7 +2463,7 @@ $("#rcmfd_new_category").keypress(function(event) {
         }
 
         if ($rsvp === null) {
-            $rsvp = !$old || $event['sequence'] > $old['sequence'];
+            $rsvp = !$old || ($event['sequence'] ?? 0) > ($old['sequence'] ?? 0);
         }
 
         $itip        = $this->load_itip();
@@ -3990,11 +3993,15 @@ $("#rcmfd_new_category").keypress(function(event) {
      */
     public function find_first_occurrence($event)
     {
-        // Make sure libkolab plugin is loaded in case of Kolab driver
+        // Make sure libkolab/libcalendaring plugins are loaded
         $this->load_driver();
 
-        // Use libkolab to compute recurring events (and libkolab plugin)
-        if (class_exists('kolabformat') && class_exists('kolabcalendaring') && class_exists('kolab_date_recurrence')) {
+        $driver_name = $this->rc->config->get('calendar_driver', 'database');
+
+        // Use kolabcalendaring/kolabformat to compute recurring events only with the Kolab driver
+        if ($driver_name == 'kolab' && class_exists('kolabformat') && class_exists('kolabcalendaring')
+            && class_exists('kolab_date_recurrence')
+        ) {
             $object = kolab_format::factory('event', 3.0);
             $object->set($event);
 
diff --git a/plugins/calendar/drivers/caldav/caldav_calendar.php b/plugins/calendar/drivers/caldav/caldav_calendar.php
index 742e9dac..7d4e94f7 100644
--- a/plugins/calendar/drivers/caldav/caldav_calendar.php
+++ b/plugins/calendar/drivers/caldav/caldav_calendar.php
@@ -163,12 +163,8 @@ class caldav_calendar extends kolab_storage_dav_folder
             }
 
             if (!empty($master)) {
-                // check for match in top-level exceptions (aka loose single occurrences)
-                if (!empty($master['_formatobj']) && ($instance = $master['_formatobj']->get_instance($instance_id))) {
-                    $this->events[$id] = $this->_to_driver_event($instance, false, true, $master);
-                }
                 // check for match on the first instance already
-                else if (!empty($master['_instance']) && $master['_instance'] == $instance_id) {
+                if (!empty($master['_instance']) && $master['_instance'] == $instance_id) {
                     $this->events[$id] = $master;
                 }
                 else if (!empty($master['recurrence'])) {
@@ -580,12 +576,6 @@ class caldav_calendar extends kolab_storage_dav_folder
      */
     public function get_recurring_events($event, $start, $end = null, $event_id = null, $limit = null)
     {
-        $object = $event['_formatobj'];
-
-        if (!is_object($object)) {
-            return [];
-        }
-
         // determine a reasonable end date if none given
         if (!$end) {
             $end = clone $event['start'];
@@ -641,7 +631,8 @@ class caldav_calendar extends kolab_storage_dav_folder
         }
 
         // Check first occurrence, it might have been moved
-        if ($first = $exdata[$event['start']->format('Ymd')]) {
+        if (!empty($exdata[$event['start']->format('Ymd')])) {
+            $first = $exdata[$event['start']->format('Ymd')];
             // return it only if not already in the result, but in the requested period
             if (!($event['start'] <= $end && $event['end'] >= $start)
                 && ($first['start'] <= $end && $first['end'] >= $start)
@@ -655,7 +646,7 @@ class caldav_calendar extends kolab_storage_dav_folder
         }
 
         // use libkolab to compute recurring events
-        $recurrence = new kolab_date_recurrence($object);
+        $recurrence = libcalendaring::get_recurrence($event);
 
         $i = 0;
         while ($next_event = $recurrence->next_instance()) {
@@ -663,9 +654,7 @@ class caldav_calendar extends kolab_storage_dav_folder
             $instance_id = $next_event['start']->format($recurrence_id_format);
 
             // use this event data for future recurring instances
-            if (!empty($futuredata[$datestr])) {
-                $overlay_data = $futuredata[$datestr];
-            }
+            $overlay_data = $futuredata[$datestr] ?? null;
 
             $rec_id      = $event['uid'] . '-' . $instance_id;
             $exception   = !empty($exdata[$datestr]) ? $exdata[$datestr] : $overlay_data;
@@ -762,17 +751,10 @@ class caldav_calendar extends kolab_storage_dav_folder
         // clean up exception data
         if (!empty($record['recurrence']) && !empty($record['recurrence']['EXCEPTIONS'])) {
             array_walk($record['recurrence']['EXCEPTIONS'], function(&$exception) {
-                unset($exception['_mailbox'], $exception['_msguid'], $exception['_formatobj'], $exception['_attachments']);
+                unset($exception['_attachments']);
             });
         }
 
-        // Load the given event data into a libkolabxml container
-        // it's needed for recurrence resolving, which uses libcalendaring
-        // TODO: Drop dependency on libkolabxml?
-        $event_xml = new kolab_format_event();
-        $event_xml->set($record);
-        $record['_formatobj'] = $event_xml;
-
         return $record;
     }
 
@@ -829,10 +811,7 @@ class caldav_calendar extends kolab_storage_dav_folder
 
         // clean up exception data
         if (!empty($event['exceptions'])) {
-            array_walk($event['exceptions'], function(&$exception) use ($cleanup_fn) {
-                unset($exception['_mailbox'], $exception['_msguid'], $exception['_formatobj']);
-                $cleanup_fn($exception);
-            });
+            array_walk($event['exceptions'], $cleanup_fn);
         }
 
         // copy meta data (starting with _) from old object
diff --git a/plugins/calendar/drivers/caldav/caldav_driver.php b/plugins/calendar/drivers/caldav/caldav_driver.php
index 3ca6bfdd..f5f91c31 100644
--- a/plugins/calendar/drivers/caldav/caldav_driver.php
+++ b/plugins/calendar/drivers/caldav/caldav_driver.php
@@ -34,6 +34,8 @@ class caldav_driver extends kolab_driver
     public $alarm_types         = ['DISPLAY', 'AUDIO'];
     public $categoriesimmutable = true;
 
+    protected $scheduling_properties = ['start', 'end', 'location'];
+
     /**
      * Default constructor
      */
@@ -530,11 +532,6 @@ class caldav_driver extends kolab_driver
      */
     public function get_recurring_events($event, $start, $end = null)
     {
-        // load the given event data into a libkolabxml container
-        $event_xml = new kolab_format_event();
-        $event_xml->set($event);
-        $event['_formatobj'] = $event_xml;
-
         $this->_read_calendars();
         $storage = reset($this->calendars);
 
@@ -546,13 +543,8 @@ class caldav_driver extends kolab_driver
      */
     protected function get_recurrence_count($event, $dtstart)
     {
-        // load the given event data into a libkolabxml container
-        $event_xml = new kolab_format_event();
-        $event_xml->set($event);
-        $event['_formatobj'] = $event_xml;
-
         // use libkolab to compute recurring events
-        $recurrence = new kolab_date_recurrence($event['_formatobj']);
+        $recurrence = libcalendaring::get_recurrence($event);
 
         $count = 0;
         while (($next_event = $recurrence->next_instance()) && $next_event['start'] <= $dtstart && $count < 1000) {
@@ -562,6 +554,98 @@ class caldav_driver extends kolab_driver
         return $count;
     }
 
+    /**
+     * Determine whether the current change affects scheduling and reset attendee status accordingly
+     */
+    protected function check_scheduling(&$event, $old, $update = true)
+    {
+        // skip this check when importing iCal/iTip events
+        if (isset($event['sequence']) || !empty($event['_method'])) {
+            return false;
+        }
+
+        // iterate through the list of properties considered 'significant' for scheduling
+        $reschedule = $this->is_rescheduling_needed($event, $old);
+
+        // reset all attendee status to needs-action (#4360)
+        if ($update && $reschedule && !empty($event['attendees'])) {
+            $is_organizer = false;
+            $emails       = $this->cal->get_user_emails();
+            $attendees    = $event['attendees'];
+
+            foreach ($attendees as $i => $attendee) {
+                if ($attendee['role'] == 'ORGANIZER'
+                    && !empty($attendee['email'])
+                    && in_array(strtolower($attendee['email']), $emails)
+                ) {
+                    $is_organizer = true;
+                }
+                else if ($attendee['role'] != 'ORGANIZER'
+                    && $attendee['role'] != 'NON-PARTICIPANT'
+                    && $attendee['status'] != 'DELEGATED'
+                ) {
+                    $attendees[$i]['status'] = 'NEEDS-ACTION';
+                    $attendees[$i]['rsvp'] = true;
+                }
+            }
+
+            // update attendees only if I'm the organizer
+            if ($is_organizer || (!empty($event['organizer']) && in_array(strtolower($event['organizer']['email']), $emails))) {
+                $event['attendees'] = $attendees;
+            }
+        }
+
+        return $reschedule;
+    }
+
+    /**
+     * Identify changes considered relevant for scheduling
+     * 
+     * @param array Hash array with NEW object properties
+     * @param array Hash array with OLD object properties
+     *
+     * @return bool True if changes affect scheduling, False otherwise
+     */
+    protected function is_rescheduling_needed($object, $old = null)
+    {
+        $reschedule = false;
+
+        foreach ($this->scheduling_properties as $prop) {
+            $a = $old[$prop] ?? null;
+            $b = $object[$prop] ?? null;
+
+            if (!empty($object['allday'])
+                && ($prop == 'start' || $prop == 'end')
+                && $a instanceof DateTimeInterface
+                && $b instanceof DateTimeInterface
+            ) {
+                $a = $a->format('Y-m-d');
+                $b = $b->format('Y-m-d');
+            }
+
+            if ($prop == 'recurrence' && is_array($a) && is_array($b)) {
+                unset($a['EXCEPTIONS'], $b['EXCEPTIONS']);
+                $a = array_filter($a);
+                $b = array_filter($b);
+
+                // advanced rrule comparison: no rescheduling if series was shortened
+                if ($a['COUNT'] && $b['COUNT'] && $b['COUNT'] < $a['COUNT']) {
+                    unset($a['COUNT'], $b['COUNT']);
+                }
+                else if ($a['UNTIL'] && $b['UNTIL'] && $b['UNTIL'] < $a['UNTIL']) {
+                    unset($a['UNTIL'], $b['UNTIL']);
+                }
+            }
+
+            if ($a != $b) {
+                $reschedule = true;
+                break;
+            }
+        }
+
+        return $reschedule;
+    }
+
     /**
      * Callback function to produce driver-specific calendar create/edit form
      *
diff --git a/plugins/calendar/drivers/kolab/kolab_calendar.php b/plugins/calendar/drivers/kolab/kolab_calendar.php
index e0129c0a..408e4d7f 100644
--- a/plugins/calendar/drivers/kolab/kolab_calendar.php
+++ b/plugins/calendar/drivers/kolab/kolab_calendar.php
@@ -709,7 +709,8 @@ class kolab_calendar extends kolab_storage_folder_api
         }
 
         // Check first occurrence, it might have been moved
-        if ($first = $exdata[$event['start']->format('Ymd')]) {
+        if (!empty($exdata[$event['start']->format('Ymd')])) {
+            $first = $exdata[$event['start']->format('Ymd')];
             // return it only if not already in the result, but in the requested period
             if (!($event['start'] <= $end && $event['end'] >= $start)
                 && ($first['start'] <= $end && $first['end'] >= $start)
@@ -731,9 +732,7 @@ class kolab_calendar extends kolab_storage_folder_api
             $instance_id = $next_event['start']->format($recurrence_id_format);
 
             // use this event data for future recurring instances
-            if (!empty($futuredata[$datestr])) {
-                $overlay_data = $futuredata[$datestr];
-            }
+            $overlay_data = $futuredata[$datestr] ?? null;
 
             $rec_id      = $event['uid'] . '-' . $instance_id;
             $exception   = !empty($exdata[$datestr]) ? $exdata[$datestr] : $overlay_data;
diff --git a/plugins/calendar/drivers/kolab/kolab_driver.php b/plugins/calendar/drivers/kolab/kolab_driver.php
index 4062bb61..31ac42eb 100644
--- a/plugins/calendar/drivers/kolab/kolab_driver.php
+++ b/plugins/calendar/drivers/kolab/kolab_driver.php
@@ -1297,8 +1297,8 @@ class kolab_driver extends calendar_driver
 
             // when saving an instance in 'all' mode, copy recurrence exceptions over
             if (!empty($old['recurrence_id'])) {
-                $event['recurrence']['EXCEPTIONS'] = $master['recurrence']['EXCEPTIONS'];
-                $event['recurrence']['EXDATE']     = $master['recurrence']['EXDATE'];
+                $event['recurrence']['EXCEPTIONS'] = $master['recurrence']['EXCEPTIONS'] ?? [];
+                $event['recurrence']['EXDATE']     = $master['recurrence']['EXDATE'] ?? [];
             }
             else if (!empty($master['_instance'])) {
                 $event['_instance']       = $master['_instance'];
@@ -1388,7 +1388,7 @@ class kolab_driver extends calendar_driver
     /**
      * Determine whether the current change affects scheduling and reset attendee status accordingly
      */
-    public function check_scheduling(&$event, $old, $update = true)
+    protected function check_scheduling(&$event, $old, $update = true)
     {
         // skip this check when importing iCal/iTip events
         if (isset($event['sequence']) || !empty($event['_method'])) {
@@ -2235,7 +2235,9 @@ class kolab_driver extends calendar_driver
 
             foreach ($event['attendees'] as $attendee) {
                 if (in_array($attendee['email'], $user_emails)) {
-                    $partstat = $attendee['status'];
+                    if (!empty($attendee['status'])) {
+                        $partstat = $attendee['status'];
+                    }
                     break;
                 }
             }
diff --git a/plugins/calendar/lib/calendar_itip.php b/plugins/calendar/lib/calendar_itip.php
index 4f7f382f..4940fc32 100644
--- a/plugins/calendar/lib/calendar_itip.php
+++ b/plugins/calendar/lib/calendar_itip.php
@@ -1,7 +1,5 @@
 <?php
 
-require_once realpath(__DIR__ . '/../../libcalendaring/lib/libcalendaring_itip.php');
-
 /**
  * iTIP functions for the Calendar plugin
  *
@@ -28,6 +26,8 @@ require_once realpath(__DIR__ . '/../../libcalendaring/lib/libcalendaring_itip.p
  */
 class calendar_itip extends libcalendaring_itip
 {
+    protected $db_itipinvitations;
+
     /**
      * Constructor to set text domain to calendar
      */
diff --git a/plugins/kolab_addressbook/drivers/carddav/carddav_contacts.php b/plugins/kolab_addressbook/drivers/carddav/carddav_contacts.php
index 9ec2d117..b815064c 100644
--- a/plugins/kolab_addressbook/drivers/carddav/carddav_contacts.php
+++ b/plugins/kolab_addressbook/drivers/carddav/carddav_contacts.php
@@ -332,7 +332,7 @@ class carddav_contacts extends rcube_addressbook
                 $this->sortindex = array_merge($this->sortindex, $local_sortindex);
             }
         }
-        else if (is_array($this->filter['ids'])) {
+        else if (isset($this->filter['ids']) && is_array($this->filter['ids'])) {
             $ids = $this->filter['ids'];
             if (count($ids)) {
                 $uids = array_map([$this, 'id2uid'], $this->filter['ids']);
@@ -1109,21 +1109,23 @@ class carddav_contacts extends rcube_addressbook
 
         switch ($this->sort_col) {
         case 'name':
-            $str = $rec['name'] . $rec['prefix'];
+            $str = ($rec['name'] ?? '') . ($rec['prefix'] ?? '');
         case 'firstname':
-            $str .= $rec['firstname'] . $rec['middlename'] . $rec['surname'];
+            $str .= ($rec['firstname'] ?? '') . ($rec['middlename'] ?? '') . ($rec['surname'] ?? '');
             break;
 
         case 'surname':
-            $str = $rec['surname'] . $rec['firstname'] . $rec['middlename'];
+            $str = ($rec['surname'] ?? '') . ($rec['firstname'] ?? '') . ($rec['middlename'] ?? '');
             break;
 
         default:
-            $str = $rec[$this->sort_col];
+            $str = $rec[$this->sort_col] ?? '';
             break;
         }
 
-        $str .= is_array($rec['email']) ? $rec['email'][0] : $rec['email'];
+        if (!empty($rec['email'])) {
+            $str .= is_array($rec['email']) ? $rec['email'][0] : $rec['email'];
+        }
 
         return mb_strtolower($str);
     }
diff --git a/plugins/kolab_addressbook/kolab_addressbook.php b/plugins/kolab_addressbook/kolab_addressbook.php
index 4a60c755..8a6f0729 100644
--- a/plugins/kolab_addressbook/kolab_addressbook.php
+++ b/plugins/kolab_addressbook/kolab_addressbook.php
@@ -37,6 +37,7 @@ class kolab_addressbook extends rcube_plugin
     private $sources;
     private $rc;
     private $ui;
+    private $recurrent = false;
 
     const GLOBAL_FIRST = 0;
     const PERSONAL_FIRST = 1;
@@ -221,8 +222,8 @@ class kolab_addressbook extends rcube_plugin
 
         $out .= $kolab . $spec;
 
-        $this->rc->output->set_env('contactgroups', array_filter($jsdata, function($src){ return $src['type'] == 'group'; }));
-        $this->rc->output->set_env('address_sources', array_filter($jsdata, function($src){ return $src['type'] != 'group'; }));
+        $this->rc->output->set_env('contactgroups', array_filter($jsdata, function($src){ return isset($src['type']) && $src['type'] == 'group'; }));
+        $this->rc->output->set_env('address_sources', array_filter($jsdata, function($src){ return !isset($src['type']) || $src['type'] != 'group'; }));
 
         $args['content'] = html::tag('ul', $args, $out, html::$common_attrib);
         return $args;
@@ -276,27 +277,27 @@ class kolab_addressbook extends rcube_plugin
     {
         $current = rcube_utils::get_input_value('_source', rcube_utils::INPUT_GPC);
 
-        if (!$source['virtual']) {
+        if (empty($source['virtual'])) {
             $jsdata[$id] = $source;
             $jsdata[$id]['name'] = html_entity_decode($source['name'], ENT_NOQUOTES, RCUBE_CHARSET);
         }
 
         // set class name(s)
         $classes = array('addressbook');
-        if ($source['group'])
+        if (!empty($source['group']))
             $classes[] = $source['group'];
         if ($current === $id)
             $classes[] = 'selected';
-        if ($source['readonly'])
+        if (!empty($source['readonly']))
             $classes[] = 'readonly';
-        if ($source['virtual'])
+        if (!empty($source['virtual']))
             $classes[] = 'virtual';
-        if ($source['class_name'])
+        if (!empty($source['class_name']))
             $classes[] = $source['class_name'];
 
         $name = !empty($source['listname']) ? $source['listname'] : (!empty($source['name']) ? $source['name'] : $id);
         $label_id = 'kabt:' . $id;
-        $inner = ($source['virtual'] ?
+        $inner = (!empty($source['virtual']) ?
             html::a(array('tabindex' => '0'), $name) :
             html::a(array(
                     'href' => $this->rc->url(array('_source' => $id)),
@@ -331,12 +332,12 @@ class kolab_addressbook extends rcube_plugin
             return html::div(null, $inner);
         }
 
-        $out .= html::tag('li', array(
+        $out = html::tag('li', array(
                 'id' => 'rcmli' . rcube_utils::html_identifier($id, true),
                 'class' => join(' ', $classes), 
                 'noclose' => true,
             ),
-            html::div($source['subscribed'] ? 'subscribed' : null, $inner)
+            html::div(!empty($source['subscribed']) ? 'subscribed' : null, $inner)
         );
 
         $groupdata = array('out' => '', 'jsdata' => $jsdata, 'source' => $id);
diff --git a/plugins/kolab_addressbook/lib/kolab_addressbook_ui.php b/plugins/kolab_addressbook/lib/kolab_addressbook_ui.php
index f1423b09..ff5b5121 100644
--- a/plugins/kolab_addressbook/lib/kolab_addressbook_ui.php
+++ b/plugins/kolab_addressbook/lib/kolab_addressbook_ui.php
@@ -89,7 +89,6 @@ class kolab_addressbook_ui
                 $content = html::tag('li', $idx ? null : array('class' => 'separator_above'),
                     $this->plugin->api->output->button(array(
                         'label'    => 'kolab_addressbook.'.str_replace('-', '', $command),
-                        'domain'   => $this->ID,
                         'class'    => str_replace('-', ' ', $command) . ' disabled',
                         'classact' => str_replace('-', ' ', $command) . ' active',
                         'command'  => $command,
diff --git a/plugins/libcalendaring/lib/libcalendaring_recurrence.php b/plugins/libcalendaring/lib/libcalendaring_recurrence.php
index 4de96812..df802b7b 100644
--- a/plugins/libcalendaring/lib/libcalendaring_recurrence.php
+++ b/plugins/libcalendaring/lib/libcalendaring_recurrence.php
@@ -52,8 +52,6 @@ class libcalendaring_recurrence
                 $this->duration = $event['start']->diff($event['end']);
             }
 
-            $event['start']->_dateonly = !empty($event['allday']);
-
             $this->init($event['recurrence'], $event['start']);
         }
     }
@@ -67,7 +65,7 @@ class libcalendaring_recurrence
     public function init($recurrence, $start)
     {
         $this->start      = $start;
-        $this->dateonly   = !empty($start->_dateonly);
+        $this->dateonly   = !empty($start->_dateonly) || !empty($this->event['allday']);
         $this->recurrence = $recurrence;
 
         $event = [
@@ -160,6 +158,7 @@ class libcalendaring_recurrence
             $end->add(new DateInterval('P100Y'));
         }
 */
+
         return isset($end) ? $this->toDateTime($end) : false;
     }
 
diff --git a/plugins/libcalendaring/lib/libcalendaring_vcalendar.php b/plugins/libcalendaring/lib/libcalendaring_vcalendar.php
index b801981b..55420e43 100644
--- a/plugins/libcalendaring/lib/libcalendaring_vcalendar.php
+++ b/plugins/libcalendaring/lib/libcalendaring_vcalendar.php
@@ -582,8 +582,8 @@ class libcalendaring_vcalendar implements Iterator
                 if (substr($value, 0, 4) == 'http' && !strpos($value, ':attachment:')) {
                     $event['links'][] = $value;
                 }
-                else if (strlen($value) && strtoupper($params['VALUE']) == 'BINARY') {
-                    $attachment = self::map_keys($params, array('FMTTYPE' => 'mimetype', 'X-LABEL' => 'name', 'X-APPLE-FILENAME' => 'name'));
+                else if (is_string($value) && strlen($value) && !empty($params['VALUE']) && strtoupper($params['VALUE']) == 'BINARY') {
+                    $attachment = self::map_keys($params, ['FMTTYPE' => 'mimetype', 'X-LABEL' => 'name', 'X-APPLE-FILENAME' => 'name']);
                     $attachment['data'] = $value;
                     $attachment['size'] = strlen($value);
                     $event['attachments'][] = $attachment;
diff --git a/plugins/libcalendaring/libcalendaring.php b/plugins/libcalendaring/libcalendaring.php
index 38884b81..76a57e49 100644
--- a/plugins/libcalendaring/libcalendaring.php
+++ b/plugins/libcalendaring/libcalendaring.php
@@ -36,7 +36,7 @@ class libcalendaring extends rcube_plugin
     public $gmt_offset;
     public $dst_active;
     public $timezone_offset;
-    public $ical_parts = array();
+    public $ical_parts = [];
     public $ical_message;
 
     public $defaults = array(
@@ -174,10 +174,10 @@ class libcalendaring extends rcube_plugin
     /**
      * Load recurrence computation engine
      */
-    public static function get_recurrence()
+    public static function get_recurrence($object = null)
     {
         $self = self::get_instance();
-        return new libcalendaring_recurrence($self);
+        return new libcalendaring_recurrence($self, $object);
     }
 
     /**
@@ -1295,7 +1295,7 @@ class libcalendaring extends rcube_plugin
     public static function identify_recurrence_instance(&$object)
     {
         // for savemode=all, remove recurrence instance identifiers
-        if (!empty($object['_savemode']) && $object['_savemode'] == 'all' && $object['recurrence']) {
+        if (!empty($object['_savemode']) && $object['_savemode'] == 'all' && !empty($object['recurrence'])) {
             unset($object['_instance'], $object['recurrence_date']);
         }
         // set instance and 'savemode' according to recurrence-id
diff --git a/plugins/libkolab/lib/kolab_format.php b/plugins/libkolab/lib/kolab_format.php
index 325dd2c0..9f7c537f 100644
--- a/plugins/libkolab/lib/kolab_format.php
+++ b/plugins/libkolab/lib/kolab_format.php
@@ -739,7 +739,7 @@ abstract class kolab_format
      */
     public static function merge_attachments(&$object, $old)
     {
-        $object['_attachments'] = (array) $old['_attachments'];
+        $object['_attachments'] = isset($old['_attachments']) && is_array($old['_attachments']) ? $old['_attachments'] : [];
 
         // delete existing attachment(s)
         if (!empty($object['deleted_attachments'])) {
@@ -771,7 +771,7 @@ abstract class kolab_format
             else {
                 // find attachment by name, so we can update it if exists
                 // and make sure there are no duplicates
-                foreach ((array) $object['_attachments'] as $cid => $att) {
+                foreach ($object['_attachments'] as $cid => $att) {
                     if ($att && $attachment['name'] == $att['name']) {
                         $key = $cid;
                     }
diff --git a/plugins/libkolab/lib/kolab_storage_cache.php b/plugins/libkolab/lib/kolab_storage_cache.php
index ff5bd86e..e38c4190 100644
--- a/plugins/libkolab/lib/kolab_storage_cache.php
+++ b/plugins/libkolab/lib/kolab_storage_cache.php
@@ -49,6 +49,7 @@ class kolab_storage_cache
     protected $limit = null;
     protected $error = 0;
     protected $server_timezone;
+    protected $sync_start;
 
 
     /**
@@ -769,6 +770,7 @@ class kolab_storage_cache
     public function select($query = array(), $uids = false, $fast = false)
     {
         $result = $uids ? array() : new kolab_storage_dataset($this);
+        $count = null;
 
         // read from local cache DB (assume it to be synchronized)
         if ($this->ready) {
@@ -923,8 +925,12 @@ class kolab_storage_cache
     {
         if (!empty($sortcols)) {
             $sortcols = array_map(function($v) {
-                list($column, $order) = explode(' ', $v, 2);
-                return "`$column`" . ($order ? " $order" : '');
+                $v = trim($v);
+                if (strpos($v, ' ')) {
+                    list($column, $order) = explode(' ', $v, 2);
+                    return "`{$column}` {$order}";
+                }
+                return "`{$v}`";
             }, (array) $sortcols);
 
             $this->order_by = join(', ', $sortcols);
@@ -1257,13 +1263,15 @@ class kolab_storage_cache
         $write_query = "UPDATE `{$this->folders_table}` SET `synclock` = ? WHERE `folder_id` = ? AND `synclock` = ?";
 
         $max_lock_time = $this->_max_sync_lock_time();
+        $sync_lock = intval($this->metadata['synclock'] ?? 0);
 
         // wait if locked (expire locks after 10 minutes) ...
         // ... or if setting lock fails (another process meanwhile set it)
         while (
-            (intval($this->metadata['synclock']) + $max_lock_time > time()) ||
-            (($res = $this->db->query($write_query, time(), $this->folder_id, intval($this->metadata['synclock']))) &&
-                !($affected = $this->db->affected_rows($res)))
+            ($sync_lock + $max_lock_time > time()) ||
+            (($res = $this->db->query($write_query, time(), $this->folder_id, $sync_lock))
+                && !($affected = $this->db->affected_rows($res))
+            )
         ) {
             usleep(500000);
             $this->metadata = $this->db->fetch_assoc($this->db->query($read_query, $this->folder_id));
diff --git a/plugins/libkolab/lib/kolab_storage_cache_event.php b/plugins/libkolab/lib/kolab_storage_cache_event.php
index b324f561..5ad9a3a1 100644
--- a/plugins/libkolab/lib/kolab_storage_cache_event.php
+++ b/plugins/libkolab/lib/kolab_storage_cache_event.php
@@ -39,7 +39,7 @@ class kolab_storage_cache_event extends kolab_storage_cache
         $sql_data['dtend']   = $this->_convert_datetime($object['end']);
 
         // extend date range for recurring events
-        if ($object['recurrence'] && $object['_formatobj']) {
+        if ($object['recurrence']) {
             $recurrence = new kolab_date_recurrence($object['_formatobj']);
             $dtend = $recurrence->end() ?: new DateTime('now +100 years');
             $sql_data['dtend'] = $this->_convert_datetime($dtend);
diff --git a/plugins/libkolab/lib/kolab_storage_dav.php b/plugins/libkolab/lib/kolab_storage_dav.php
index ac313478..e71f3d0d 100644
--- a/plugins/libkolab/lib/kolab_storage_dav.php
+++ b/plugins/libkolab/lib/kolab_storage_dav.php
@@ -46,9 +46,6 @@ class kolab_storage_dav
      */
     public function setup()
     {
-        $rcmail = rcube::get_instance();
-
-        $this->config = $rcmail->config;
         $this->dav = new kolab_dav_client($this->url);
     }
 
diff --git a/plugins/libkolab/lib/kolab_storage_dav_cache.php b/plugins/libkolab/lib/kolab_storage_dav_cache.php
index 5dec0fa9..528b5a71 100644
--- a/plugins/libkolab/lib/kolab_storage_dav_cache.php
+++ b/plugins/libkolab/lib/kolab_storage_dav_cache.php
@@ -159,6 +159,7 @@ class kolab_storage_dav_cache extends kolab_storage_cache
 
         // Fetch new objects and store in DB
         if (!empty($new_index)) {
+            $i = 0;
             foreach (array_chunk($new_index, $chunk_size, true) as $chunk) {
                 $objects = $this->folder->dav->getData($this->folder->href, $this->folder->get_dav_type(), $chunk);
 
@@ -412,6 +413,7 @@ class kolab_storage_dav_cache extends kolab_storage_cache
     public function select($query = [], $uids = false, $fast = false)
     {
         $result = $uids ? [] : new kolab_storage_dataset($this);
+        $count  = null;
 
         $this->_read_folder_data();
 
diff --git a/plugins/libkolab/lib/kolab_storage_dav_cache_contact.php b/plugins/libkolab/lib/kolab_storage_dav_cache_contact.php
index ed55e21e..578002c8 100644
--- a/plugins/libkolab/lib/kolab_storage_dav_cache_contact.php
+++ b/plugins/libkolab/lib/kolab_storage_dav_cache_contact.php
@@ -36,21 +36,20 @@ class kolab_storage_dav_cache_contact extends kolab_storage_dav_cache
     protected function _serialize($object)
     {
         $sql_data = parent::_serialize($object);
-        $sql_data['type'] = $object['_type'] ?: 'contact';
+        $sql_data['type'] = !empty($object['_type']) ? $object['_type'] : 'contact';
 
         if ($sql_data['type'] == 'group' || (!empty($object['kind']) && $object['kind'] == 'group')) {
             $sql_data['type'] = 'group';
         }
 
         // columns for sorting
-        $sql_data['name']      = rcube_charset::clean($object['name'] . $object['prefix']);
-        $sql_data['firstname'] = rcube_charset::clean($object['firstname'] . $object['middlename'] . $object['surname']);
-        $sql_data['surname']   = rcube_charset::clean($object['surname']   . $object['firstname']  . $object['middlename']);
+        $sql_data['name']      = rcube_charset::clean(($object['name'] ?? '') . ($object['prefix'] ?? ''));
+        $sql_data['firstname'] = rcube_charset::clean(($object['firstname'] ?? '') . ($object['middlename'] ?? '') . ($object['surname'] ?? ''));
+        $sql_data['surname']   = rcube_charset::clean(($object['surname'] ?? '') . ($object['firstname'] ?? '') . ($object['middlename'] ?? ''));
         $sql_data['email']     = '';
 
         foreach ($object as $colname => $value) {
-            list($col, $field) = explode(':', $colname);
-            if ($col == 'email' && !empty($value)) {
+            if (!empty($value) && ($colname == 'email' || strpos($colname, 'email:') === 0)) {
                 $sql_data['email'] = is_array($value) ? $value[0] : $value;
                 break;
             }
@@ -84,14 +83,14 @@ class kolab_storage_dav_cache_contact extends kolab_storage_dav_cache
         $data = '';
 
         foreach ($object as $colname => $value) {
-            list($col, $field) = explode(':', $colname);
+            list($col, $field) = strpos($colname, ':') ? explode(':', $colname) : [$colname, null];
 
-            $val = '';
+            $val = null;
             if (in_array($col, $this->fulltext_cols)) {
                 $val = is_array($value) ? join(' ', $value) : $value;
             }
 
-            if (strlen($val)) {
+            if (is_string($val) && strlen($val)) {
                 $data .= $val . ' ';
             }
         }
diff --git a/plugins/libkolab/lib/kolab_storage_dav_cache_event.php b/plugins/libkolab/lib/kolab_storage_dav_cache_event.php
index 8e7c7e75..cd35070d 100644
--- a/plugins/libkolab/lib/kolab_storage_dav_cache_event.php
+++ b/plugins/libkolab/lib/kolab_storage_dav_cache_event.php
@@ -41,19 +41,14 @@ class kolab_storage_dav_cache_event extends kolab_storage_dav_cache
 
         // extend date range for recurring events
         if (!empty($object['recurrence'])) {
-            if (empty($object['_formatobj'])) {
-                $event_xml = new kolab_format_event();
-                $event_xml->set($object);
-                $object['_formatobj'] = $event_xml;
-            }
-
-            $recurrence = new kolab_date_recurrence($object['_formatobj']);
+            $recurrence = libcalendaring::get_recurrence($object);
             $dtend = $recurrence->end() ?: new DateTime('now +100 years');
             $sql_data['dtend'] = $this->_convert_datetime($dtend);
         }
 
         // extend start/end dates to spawn all exceptions
-        if (is_array($object['exceptions'])) {
+        // FIXME: This should be done via libcalendaring_recurrence use above?
+        if (!empty($object['exceptions']) && is_array($object['exceptions'])) {
             foreach ($object['exceptions'] as $exception) {
                 if ($exception['start'] instanceof DateTimeInterface) {
                     $exstart = $this->_convert_datetime($exception['start']);
@@ -86,12 +81,18 @@ class kolab_storage_dav_cache_event extends kolab_storage_dav_cache
         $data = '';
 
         foreach ($this->fulltext_cols as $colname) {
-            list($col, $field) = explode(':', $colname);
+            list($col, $field) = strpos($colname, ':') ? explode(':', $colname) : [$colname, null];
+
+            if (empty($object[$col])) {
+                continue;
+            }
 
             if ($field) {
                 $a = [];
                 foreach ((array) $object[$col] as $attr) {
-                    $a[] = $attr[$field];
+                    if (!empty($attr[$field])) {
+                        $a[] = $attr[$field];
+                    }
                 }
                 $val = join(' ', $a);
             }
@@ -99,14 +100,15 @@ class kolab_storage_dav_cache_event extends kolab_storage_dav_cache
                 $val = is_array($object[$col]) ? join(' ', $object[$col]) : $object[$col];
             }
 
-            if (strlen($val))
+            if (is_string($val) && strlen($val)) {
                 $data .= $val . ' ';
+            }
         }
 
         $words = rcube_utils::normalize_string($data, true);
 
         // collect words from recurrence exceptions
-        if (is_array($object['exceptions'])) {
+        if (!empty($object['exceptions']) && is_array($object['exceptions'])) {
             foreach ($object['exceptions'] as $exception) {
                 $words = array_merge($words, $this->get_words($exception));
             }
@@ -137,14 +139,14 @@ class kolab_storage_dav_cache_event extends kolab_storage_dav_cache
         }
 
         // collect tags from recurrence exceptions
-        if (is_array($object['exceptions'])) {
+        if (!empty($object['exceptions']) && is_array($object['exceptions'])) {
             foreach ($object['exceptions'] as $exception) {
                 $tags = array_merge($tags, $this->get_tags($exception));
             }
         }
 
         if (!empty($object['status'])) {
-          $tags[] = 'x-status:' . strtolower($object['status']);
+            $tags[] = 'x-status:' . strtolower($object['status']);
         }
 
         return array_unique($tags);