Fullscreen OpenGL applications result in session termination

[Thetargos]

Well, ever since installing XFCE 4.2 on my primary system any OpenGL
application (especially Screensavers) will for not apparent reason
terminate the session from which it was started... I don't understand
why this occurs. What puzzles me is that this did not happen right
after I installed it, but after a few days this shows up... What would
be the best way to look at what is going on?? A log or something??