diff --git a/plugins/calendar/drivers/kolab/kolab_calendar.php b/plugins/calendar/drivers/kolab/kolab_calendar.php
index d95de4e6..e09e49a8 100644
--- a/plugins/calendar/drivers/kolab/kolab_calendar.php
+++ b/plugins/calendar/drivers/kolab/kolab_calendar.php
@@ -529,6 +529,11 @@ class kolab_calendar extends kolab_storage_folder_api
             return false;
         }
 
+        // When updating an event (e.g. when restoring an event occurrence)
+        // the message UID might be outdated. Use the updated UID from database.
+        // See kolab_driver::restore_event().
+        $event['_msguid'] = $old['_msguid'];
+
         // email links are stored separately
         $links = !empty($event['links']) ? $event['links'] : [];
         unset($event['links']);
diff --git a/plugins/calendar/drivers/kolab/kolab_driver.php b/plugins/calendar/drivers/kolab/kolab_driver.php
index 4d7cf5f8..7d4f180d 100644
--- a/plugins/calendar/drivers/kolab/kolab_driver.php
+++ b/plugins/calendar/drivers/kolab/kolab_driver.php
@@ -880,7 +880,7 @@ class kolab_driver extends calendar_driver
 
             switch ($savemode) {
             case 'current':
-                $_SESSION['calendar_restore_event_data'] = $master;
+                $_SESSION['calendar_restore_event_data'] = serialize(array_diff_key($master, ['_formatobj' => 1]));
 
                 // remove the matching RDATE entry
                 if (!empty($master['recurrence']['RDATE'])) {
@@ -901,7 +901,7 @@ class kolab_driver extends calendar_driver
             case 'future':
                 $master['_instance'] = libcalendaring::recurrence_instance_identifier($master);
                 if ($master['_instance'] != $event['_instance']) {
-                    $_SESSION['calendar_restore_event_data'] = $master;
+                    $_SESSION['calendar_restore_event_data'] = serialize(array_diff_key($master, ['_formatobj' => 1]));
 
                     // set until-date on master event
                     $master['recurrence']['UNTIL'] = clone $event['start'];
@@ -984,7 +984,7 @@ class kolab_driver extends calendar_driver
     {
         if ($storage = $this->get_calendar($event['calendar'])) {
             if (!empty($_SESSION['calendar_restore_event_data'])) {
-                $success = $storage->update_event($event = $_SESSION['calendar_restore_event_data']);
+                $success = $storage->update_event($event = unserialize($_SESSION['calendar_restore_event_data']));
             }
             else {
                 $success = $storage->restore_event($event);
diff --git a/plugins/libkolab/lib/kolab_storage_cache.php b/plugins/libkolab/lib/kolab_storage_cache.php
index 7c8575e5..360688b5 100644
--- a/plugins/libkolab/lib/kolab_storage_cache.php
+++ b/plugins/libkolab/lib/kolab_storage_cache.php
@@ -533,7 +533,7 @@ class kolab_storage_cache
         }
 
         $this->check_error();
-        return $this->objects[$msguid];
+        return $this->objects[$msguid] ?? null;
     }
 
     /**