The server’s configuration works for other apps including the official NextCloud Android app and GNOME Online Accounts, so that points to a davx5 problem.
No, that message is normal. It always comes with the username/password wrong message.
We have thousands of Nextcloud users (including myself) without problems and we constantly test Nextcloud with all possible methods (Login flow and URL), and it always works.
If you can provide steps to reproduce the problem with a fresh Nextcloud installation or have specific hints what a possible DAVx5 problem could be, please let us know. For me, it looks like a username/password problem at the moment (or maybe you are locked out with our mobile IP, for instance because of too many wrong tries).
What do Nextcloud logs say? Did you clean the brute force attempts table?