Problems with Shutdown and mounting of DVD-drives

[BlueLion]

Hi there,
I've got Linux Mint 9 running and having problems with shutdown and mounting of drives since recently.

Options for shutdown and restart in the shutdown-window are grey, thus not selectable. Shutdown and restart DO work via terminal though.
What's worse is that I can't read any CDs or DVDs anymore - as soon as I try to read the drives following error message appears:
Unable to mount Disk

Not Authorized: Remote Exception invoking org.freedesktop.PolicyKit1.Authority.CheckAuthorization() on /org/freedesktop/PolicyKit1/Authority at name org.freedesktop.PolicyKit1: org.freedesktop.PolicyKit1.Error.Failed: Action org.freedesktop.udisks.filesystem-mount is not registered

I've also got Linx Mint 8 installed on my machine, it's running just fine, so the error has to be located in the Isadora-installation or Ubuntu 10.04.
The upgrade history is complete, and my user has administrative rights.
Anybody with proposals?

Kind regards
BlueLion

[BlueLion]

I want to add, that shutdown on terminal is only possible with sudo-command as well!