How to Resolve Problems in Outlook for Delegate Mailbox Users
Perform this procedure if delegated users are receiving error messages such as "Unable to open the free/busy message" when they try to modify the mailbox owner's calendar.
Procedure
How to resolve problems in Outlook for delegate mailbox users
Request that all delegate users and the mailbox owner shut down Outlook.
Request that the mailbox owner start Outlook with the /cleanfreebusy switch. The mailbox owner can do this from the command prompt by typing:
outlook/cleanfreebusy
Outlook will re-create the LocalFreeBusy message and synchronize the other mailbox folders to use it.
Resume using Outlook. Delegates should now be able to use the mailbox as expected.