No shutdown

[Minze]

Hi there, here's my latest issue with Linux Mint12: After I set up a guest account, my system won't shut down anymore. When I choose the option "shutdown" the system goes back to the initial screen where I choose the user. Also changing the accounts as admin is locked now. I tried every interface (mate, gnome) but no help. The only way to shut it down at the moment is the power switch on the computer.
I would really appreciate any help!
Thanks!