Problem solved. It seems to be caused by an old package from dri.sourceforge.net (xserver-xfree86-dri-trunk). After removing that and all corresponding packages and reinstalling the packages xserver-xfree86 and xlibmesa-dri the xserver runs fine now.
I tried 3 different Java programms with GUI and all were running without problems and I couldn't find errors in XFree86.0.log. good night, Martin
