I have ran into the same issue, as referenced in : https://docs.hetzner.com/robot/dedicated-server/windows-server/changing-windows-password/ (though it may differ based on installation).
Steps :
- Open RDP, show options & Save As
- Edit that saved file in notepad, adding these lines at the bottom :
username:s:Administrator
authentication level:i:2
enablecredsspsupport:i:0
Now load the saved file and reconnect.
Hope this helps you / whomever landed here from a search engine!