[ubuntu] All applications freeze immediatly after start, but not Ubuntu 14.04 itself

Hi there,
I've got a freshly installed Ubuntu 14.04 LTS on a Samsung SSD. Installation was smooth, the system boots without any problem and everything seems to be fine.
But I can start whatever I want (Firefox, LibreOffice, Terminal...), every application starts up and goes in a freeze. For example, Firefox starts up and goes straight into the crash reporter. LibreOffice starts up with its complete GUI and a blank sheet. You think you can start typing, but it hangs.

"Freeze" means: nothing happens in the app, you can't click anything, you even can't use the window controls (x, dash, square). The application window seems to be totally dead. You can kill the app by a new(!) terminal window and kill command.

Ubuntu itself is fully up and running: I can start apps from the left bar, I can change the language in the upper right corner, I can reboot the system. So Ubuntu is fully alive, but every app dies immediatly after startup. Hardware seems not to be the problem, since Ubuntu Live-CD is working fine.

Anybody any idea what to do, ...what to check?

Thanks
Frank