Ok, just found the answer by myself one second later -> if anyone has trouble with this check extended permissions / extended settings on your webdav folder.
Cheers
I have the same issue since 2 months with syncing my webdav folder on my synology NAS via DynDNS.
For years I had no problems at all. It's the same on all my devices IOS and Windows. Did you/anyone found an answer to solve this?