diff --git a/lib/kolab_sync_data.php b/lib/kolab_sync_data.php
--- a/lib/kolab_sync_data.php
+++ b/lib/kolab_sync_data.php
@@ -751,7 +751,7 @@
                 $data[$relation['uid']] = array(
                     'name'    => $relation['name'],
                     'changed' => $relation['changed']->format('U'),
-                    'members' => implode("\n", $relation['members']),
+                    'members' => implode("\n", (array)$relation['members']),
                 );
             }