Exchange 5.5 loses ownership permissions

We have a frequent problem with Exchange 5.5 on a Win2k server where the users are losing ownership permissions of their mailbox – and are getting the message “Unable to display the folder – you do not have permission to logon.” when opening Outlookand selecting their Personal folder. They are successfully logging into the file server, which is also win2k. The problem is temporarily corrected when we re-assign the user as mailbox owner – and reboot the server – only to lose another user. No consistancies have been found.

We’re thinking it’s an issue with Active Directory or the Active Directory Connector – but uncertain of the solution. Any ideas would be greatly appreciated – we’re perplexed.