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
@@ -803,21 +803,60 @@
             throw new Syncroton_Exception_Status_MeetingResponse(Syncroton_Exception_Status_MeetingResponse::INVALID_REQUEST);
         }
 
-        if ($event['free_busy']) {
-            $old['free_busy'] = $event['free_busy'];
+        // A single recurring event occurrence
+        if (!empty($event['recurrence_date'])) {
+            $event['recurrence'] = [];
+
+            if ($status) {
+                $this->update_attendee_status($event, $status);
+                $status = null;
+            }
+
+            if (!isset($old['exceptions'])) {
+                if (isset($old['recurrence']['EXCEPTIONS'])) {
+                    $old['exceptions'] = &$old['recurrence']['EXCEPTIONS'];
+                }
+                else {
+                    $old['exceptions'] = [];
+                }
+            }
+
+            $existing = false;
+            foreach ($old['exceptions'] as $i => $exception) {
+                if (libcalendaring::is_recurrence_exception($event, $exception)) {
+                    $old['exceptions'][$i] = $event;
+                    $existing = true;
+                }
+            }
+
+            if (!$existing) {
+                $old['exceptions'][] = $event;
+            }
+        }
+        // A main event update
+        else if (isset($event['sequence']) && $event['sequence'] > $old['sequence']) {
+            // FIXME: Can we be smarter here? Should we update everything? What about e.g. new attendees?
+            //        And do we need to check the sequence?
+            $props = ['start', 'end', 'title', 'description', 'location', 'free_busy'];
+
+            foreach ($props as $prop) {
+                if (isset($event[$prop])) {
+                    $old[$prop] = $event[$prop];
+                }
+            }
+
+            // Copy new custom properties
+            if (!empty($event['x-custom'])) {
+                foreach ($event['x-custom'] as $key => $val) {
+                    $old['x-custom'][$key] = $val;
+                }
+            }
         }
 
         // Updating an existing event is most-likely a response
         // to an iTip request with bumped SEQUENCE
         $old['sequence'] += 1;
 
-        // Copy new custom properties
-        if (!empty($event['x-custom'])) {
-            foreach ($event['x-custom'] as $key => $val) {
-                $old['x-custom'][$key] = $val;
-            }
-        }
-
         // Update the event
         return $this->save_event($old, $status);
     }