How did you install the Windows Server? In the Windows setup process, you should have set up an admin user . You will need to use this user, not the one Netcup gave you probably with the default Linux installation.
Have you added the user to the Remote Desktop User group? in Computer Management? And incoming RDP must be allowed in the firewall.
If anything: DO NOT USE the "Administrator" account!!! Add a user in Computer Management, add it to the Administrators group, then log in as that user and deactivate the "Administrator". Your server will be attacked permanently on the RDP port (if not behind a firewall) and those attacks will all try to use "Administrator".
It indeed would be best, if the RDP port would not be open to the internet, but only to VPN interfaces like Zerotier or Tailscale.