It seems that this was a configuration problem that did not have any effect on earlier versions of Ubuntu. It disappeared after reinstalling GRUB2. It seems that the additional parameter "text" in the kernel arguments (see the line for "ProcKernelCmdLine" above) caused this problem on my system.
** Changed in: xorg (Ubuntu) Status: New => 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/1407991 Title: Xorg freeze To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/1407991/+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