It seems that the iOS app is stuck in synchronizing. I had 16 photos in the queue to be synchronized to my main server. When I tried to change the server from the header, I always got the info, that there is a running synchronization (“Laufende Übertragung, bitte warten…”). But no photos were synchronized. I canceled the synchronization for each file and restarted then manually. After that the app synchronized and I was able to change the server.