I have the same problem on a rls 4.1
The problem is when a user press the visual voicemail key on a 5220 ip phone.
If he doesn't exit correctly by pressing exit of the visual voicemail. I don't have the solution, but a least they know what to do if they don't want their mailbox to be in this state.
The big problem with this is that you cannot delete the mailbox or even do a backup of the system.
The only way to fix it is a reboot.