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
@@ -490,6 +490,7 @@
         // this update resets attendee status set in the MeetingResponse request.
         // We ignore changes to attendees data on such updates
         if ($is_outlook && $this->isDummyOutlookUpdate($data, $entry, $event)) {
+            $this->logger->debug('Dummy outlook update detected, ignoring attendee changes.');
             $attendees = $entry['attendees'];
         }
         else if (isset($data->attendees)) {