diff --git a/plugins/calendar/calendar.php b/plugins/calendar/calendar.php
index f08d8f8e..f3e8b3c6 100644
--- a/plugins/calendar/calendar.php
+++ b/plugins/calendar/calendar.php
@@ -2115,22 +2115,24 @@ $("#rcmfd_new_category").keypress(function(event) {
 
         // check for organizer in attendees list
         $organizer = null;
-        foreach ((array) $event['attendees'] as $i => $attendee) {
-            if (!empty($attendee['role']) && $attendee['role'] == 'ORGANIZER') {
-                $organizer = $attendee;
-            }
-            if (!empty($attendee['status']) && $attendee['status'] == 'DELEGATED' && empty($attendee['rsvp'])) {
-                $event['attendees'][$i]['noreply'] = true;
-            }
-            else {
-                unset($event['attendees'][$i]['noreply']);
+        if (!empty($event['attendees'])) {
+            foreach ((array) $event['attendees'] as $i => $attendee) {
+                if (!empty($attendee['role']) && $attendee['role'] == 'ORGANIZER') {
+                    $organizer = $attendee;
+                }
+                if (!empty($attendee['status']) && $attendee['status'] == 'DELEGATED' && empty($attendee['rsvp'])) {
+                    $event['attendees'][$i]['noreply'] = true;
+                }
+                else {
+                    unset($event['attendees'][$i]['noreply']);
+                }
             }
         }
 
         if ($organizer === null && !empty($event['organizer'])) {
             $organizer = $event['organizer'];
             $organizer['role'] = 'ORGANIZER';
-            if (!is_array($event['attendees'])) {
+            if (!isset($event['attendees']) || !is_array($event['attendees'])) {
                 $event['attendees'] = [$organizer];
             }
         }
diff --git a/plugins/calendar/drivers/calendar_driver.php b/plugins/calendar/drivers/calendar_driver.php
index 8bcd6903..49c79579 100644
--- a/plugins/calendar/drivers/calendar_driver.php
+++ b/plugins/calendar/drivers/calendar_driver.php
@@ -507,7 +507,7 @@ abstract class calendar_driver
                 // add to output if in range
                 if (($next_event['start'] <= $end && $next_event['end'] >= $start)) {
                     $next_event['_instance'] = $next_event['start']->format($recurrence_id_format);
-                    $next_event['id'] = $next_event['uid'] . '-' . $exception['_instance'];
+                    $next_event['id'] = $next_event['uid'] . '-' . $next_event['_instance'];
                     $next_event['recurrence_id'] = $event['uid'];
                     $events[] = $next_event;
                 }
@@ -647,7 +647,7 @@ abstract class calendar_driver
         $year2  = $end->format('Y');
 
         $events = [];
-        $search = mb_strtolower($search);
+        $search = mb_strtolower((string) $search);
         $rcmail = rcmail::get_instance();
         $cache  = $rcmail->get_cache('calendar.birthdays', 'db', 3600);
         $cache->expunge();
@@ -786,18 +786,14 @@ abstract class calendar_driver
         }
 
         try {
-            $bday = $contact['birthday'];
-            if (!$bday instanceof DateTimeInterface) {
-                $bday = new DateTime($bday, new DateTimeZone('UTC'));
-            }
-            $bday->_dateonly = true;
+            $bday = libcalendaring_datetime::createFromAny($contact['birthday'], true);
         }
         catch (Exception $e) {
             rcube::raise_error([
                     'code' => 600,
                     'file' => __FILE__,
                     'line' => __LINE__,
-                    'message' => 'BIRTHDAY PARSE ERROR: ' . $e->getMessage()
+                    'message' => 'Failed to parse contact birthday: ' . $e->getMessage()
                 ],
                 true, false
             );
diff --git a/plugins/libcalendaring/lib/libcalendaring_datetime.php b/plugins/libcalendaring/lib/libcalendaring_datetime.php
index 95bdb94e..9f548988 100644
--- a/plugins/libcalendaring/lib/libcalendaring_datetime.php
+++ b/plugins/libcalendaring/lib/libcalendaring_datetime.php
@@ -26,4 +26,30 @@
 class libcalendaring_datetime extends DateTime
 {
     public $_dateonly = false;
+
+    /**
+     * Create an instance from a date string or object
+     *
+     * @param DateTimeInterface|string $date     Date
+     * @param bool                     $dateonly Date only (ignore time)
+     */
+    public static function createFromAny($date, bool $dateonly = false)
+    {
+        if (!$date instanceof DateTimeInterface) {
+            $date = new DateTime($date, new DateTimeZone('UTC'));
+        }
+
+        // Note: On PHP8 we have DateTime::createFromInterface(), but not on PHP7
+
+        $result = self::createFromFormat(
+            'Y-m-d\\TH:i:s',
+            $date->format('Y-m-d\\TH:i:s'),
+            // Sabre will loose timezone on all-day events, use the event start's timezone
+            $date->getTimezone()
+        );
+
+        $result->_dateonly = $dateonly;
+
+        return $result;
+    }
 }
diff --git a/plugins/libcalendaring/libcalendaring.php b/plugins/libcalendaring/libcalendaring.php
index 291aa270..e2de7f2f 100644
--- a/plugins/libcalendaring/libcalendaring.php
+++ b/plugins/libcalendaring/libcalendaring.php
@@ -853,7 +853,7 @@ class libcalendaring extends rcube_plugin
             return $this->gettext('ondate') . ' ' . join(', ', $rdates) . ($more ? '...' : '');
         }
 
-        $output  = sprintf('%s %d ', $this->gettext('every'), $rrule['INTERVAL'] ?: 1);
+        $output  = sprintf('%s %d ', $this->gettext('every'), !empty($rrule['INTERVAL']) ? $rrule['INTERVAL'] : 1);
 
         switch ($rrule['FREQ']) {
         case 'DAILY':