@rfc2822 Yes, that sounds exactly like my problem. Sorry that I didn’t find this before, I wouldn’t have opened another topic then.
What’s strange though, changing the label of a card on Android still gets synced back to the server. I really wonder how that’s possible, if the PUT request is denied in the first place.
Never mind, at least I now know that there is nothing wrong with my setup. And for everybody else, eventually ending up here, it looks like the issue is handled at https://github.com/nextcloud/deck/issues/2355