some update: I removed and reinstalled the driver and checked my xorg.0.log for other errors, but no results..
edit:
I disabled AIGLX and composite. The messages still appears, but xorg keeps going. I can work with that for now. What do the messages mean?
edit2:
That didn't help, after many restarts during that session, something stopped that caused the trouble. Currently I have disabled atieventsd and after some reboots I'm almost convinced all the troubles were caused by it again!