@rfc2822 Thanks a lot for your quick reply! You were right: Having edited the affected entry in the nextcloud GUI, subsequent edits one the mobile phone synced without a problem.
Thanks again! Christoph
Nextcloud has some kind of flood protection - if the pw was entered wrong for a few times, it does not accept requests for a period of time (minutes I guess).
Another thing : Did you enable app specific passwords and two step verification? If yes, you need this password to login and not the one you use to login to Nextcloud!
I’m having the same problem but the app password was entered correctly the first time. But CalDav says it can’t find any DAV service .
See: link Link
@flux84 Does it work for you now?