HI! Angol fórumon találtam, egy próbát megér:
CTRL + ALT + F2-be ezteket:
sudo /etc/init.d/gdm stop
startx
did the trick, but in order to access Administration -> Login screen, I had to start gdm. Then I was able to make the changes needed. Now the system is back in business again.
Actually, I pressed ALT+CTRL+F2, and that opened tty2. I entered all the commands from here, and the black screen with the spinning mouse-pointer was back. Pressing ALT+CTRL+F7 brought me back to the x session, and now when gdm was started, I could change anything I liked. BUT ... With every change I made, the "bad-screen" came back. I figured that I was thrown back to tty2, so I just pressed ALT+CTRL+F7 and could continue to change settings. After a restart, everything was normal.
Thanks again!