OK, I have done something terribly stupid.
On the domain controler, I have was playing with a new group policy for our new terminal server. As you can understand, I limited as much as possible (cmd, powershell, regedit, etc). Stupid me, I also restricted the administrator user.
Now, when logging in to the server or any other for that matter, it blocks all. I can not remove this policy. What can I do to temporarily disable GPO, and unset it (at least for admin users)?