Thanks for your suggestion but bumblebee was not installed. However, after switching the driver to the nvidia-update Branch and the Destkop- Manager from GNOME to Unity, the problem vanished. So this might as well be a GNOME problem, thus i'm marking it as invalid.
** Changed in: xorg (Ubuntu) Status: Confirmed => Invalid -- You received this bug notification because you are a member of Ubuntu-X, which is subscribed to xorg in Ubuntu. https://bugs.launchpad.net/bugs/1300669 Title: X Server fails to start with nvidia-331 in 14.04 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/1300669/+subscriptions _______________________________________________ Mailing list: https://launchpad.net/~ubuntu-x-swat Post to : ubuntu-x-swat@lists.launchpad.net Unsubscribe : https://launchpad.net/~ubuntu-x-swat More help : https://help.launchpad.net/ListHelp