Working again after the last X-Server update yesterday.
That did work only until the next reboot.
But I found another solution here: http://ubuntuforums.org/showthread.php?t=1864672&page=2
And it works since then.
This is a problem with the nvidia installation and any Qt-Software. There is a wrong parameter set in the xorg.conf who is apparently also set automatically if there is no xorg.conf - as it was in my case.
just commenting out the following lines and restarting solved the problem: