I never thought I would be that unfocused but apparently I did... I disabled the Administrator user and created a new user with password. Unfortunatly I forgot to add this user to the remote access users group. Now I cannot login anymore to this server, which is 1500 miles away, btw. Luckily (?) we have other servers in the subnet.
I tried accessing it via regedit -> connect to remote computer; but I couldn't connect to the server with the credentials of the new (and only) user.
They are all Windows 2003 R2 Servers.
Thanks in advance!!