To verify, I tried several Nextcloud users and always got the result
Success! Synchronization configuration appears to be correct.
Under Windows I mounted a drive like:
net use Z: https:///remote.php/dav/files/USERNAME/ /user:youruser yourpassword
and uploaded a file, which works as well.
Now I found out something strange:
All users are mounted on external storage in Nextcloud and all have the same error.
When I create a new user, which is not on an external storage, the sync works fine.
Do I have to change the WebDAV URL and if, into what?