Hi again, I downgraded xserver-xorg and xserver-common to 6.8.2.dfsg.1-11. The problem is gone, so I can confirm that this bug is not related to anything else.
In case someone else is affected: Oliver Haake wrote me an email suggesting to delete /usr/X11R6/lib/modules/drivers/mga_hal.o, which solved the problem for him. Bye, Sebastian Kuhnert
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