In order to get the PC usable, I have tried to disable the part of the system causing the error. Disabling GLX made no difference (Option "GLX" "Disable"), but disabling the 2D hardware acceleration did it (Option "AccelMethod" "none"). So with the latter setting, the PC is usable until there is a solution to the hardware acceleration problem. BTW: A test with 2D acceleration and the ' Option "ShadowPrimary" "on" ' crashed the x-server, showing a backtrace on the console. Possibly caused by the same issue.
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