granting permissions on mailbox root folder generates error message

hello,

trying to grant permissions on the root folder of a mailbox ends up giving the following error message:

--> "The modified permissions cannot be saved"

Trying to do it with PS also doesn't work.

I moved also the mailbox to a different mailbox database but also not luck with that.

exchange 2010 SP 2 Rollup 8

Outlook 2007

Does anyone have an idea how to fix the problem?

Thanks, Roberto