Restart, Shut Off, don't work in 8.04

[ferdinando_villa]

I've just updated my 7.10 Gutsy Kubuntu to 8.04, but now Restart and Shut Off don't work - if i click on either one, i just get a black screen, and nothing else happens - the same happens with ctrl + alt + backspace, to start a new session without restarting - just a black screen.

Is anyone else experiencing the same problem? Any ideas on how to fix it?
Thank you