Ok, I think I can give some more information about this:
I had the same problem with the classic app on windows server 2016 in a terminal session. the strange thing was, that it already worked on the same machine. so I checked and it works if the user is in a group with administration rights but not if it is a regular user.
I also checked from the nextcloud side and it it looks like the request isn't even sent to the webdav server.
it works with admin rights
if it isn't working, no request is sent to the webdav server