diff --git a/lib/kolab_sync_data_calendar.php b/lib/kolab_sync_data_calendar.php
--- a/lib/kolab_sync_data_calendar.php
+++ b/lib/kolab_sync_data_calendar.php
@@ -391,8 +391,10 @@
         $is_outlook   = stripos($this->device->devicetype, 'outlook') !== false;
         $is_android   = stripos($this->device->devicetype, 'android') !== false;
 
-        // check data validity
-        $this->check_event($data);
+        // check data validity (of a new event)
+        if (empty($event)) {
+            $this->check_event($data);
+        }
 
         if (!empty($event['start']) && ($event['start'] instanceof DateTime)) {
             $old_timezone = $event['start']->getTimezone();
@@ -429,6 +431,11 @@
 
             $value = $data->$key;
 
+            // Skip ghosted (unset) properties, (but make sure 'changed' timestamp is reset)
+            if ($value === null && $name != 'changed') {
+                continue;
+            }
+
             switch ($name) {
             case 'changed':
                 $value = null;
@@ -510,12 +517,27 @@
         }
 
         // Organizer
-        if (!$is_exception && ($organizer_email = $data->organizerEmail)) {
-            $attendees[] = array(
-                'role'   => 'ORGANIZER',
-                'name'   => $data->organizerName,
-                'email'  => $organizer_email,
-            );
+        if (!$is_exception) {
+            // Organizer specified
+            if ($organizer_email = $data->organizerEmail) {
+                $attendees[] = array(
+                    'role'   => 'ORGANIZER',
+                    'name'   => $data->organizerName,
+                    'email'  => $organizer_email,
+                );
+            } else if (!empty($event['attendees'])) {
+                // Organizer not specified, use one from the original event if that's an update
+                foreach ($event['attendees'] as $idx => $attendee) {
+                    if (!empty($attendee['email']) && !empty($attendee['role']) && $attendee['role'] == 'ORGANIZER') {
+                        $organizer_email = $attendee['email'];
+                        $attendees[] = array(
+                            'role'   => 'ORGANIZER',
+                            'name'   => $attendee['name'] ?? '',
+                            'email'  => $organizer_email,
+                        );
+                    }
+                }
+            }
         }
 
         // Attendees