Some controls aren't valid - Changes in the user/group........

We migrated from Exchange 2003 to 2010 several months ago. I've had a couple of reports from some people not being able to modify delegate permissions. So from some investigation into mailbox accounts I am receiing the above error whenever I add, then manage 'Full Mailbox Permissions'.

Having searched on here I've noted and performed the followijg actions

The error 'Some controls aren't valid - Changes in the user/group........' happens for either migtated or new accounts.

I have checked the alias for spaces

Rebooted the Exchange Server

None of the above has stopped the error so does anyone have anty further idea?

Thanks