Unable to book resources in Outlook XP

Currently running Exchange 5.5 on an NT network with Outlook XP. When some of my users try to book a couple of the resources it gives them an error message:

“The operation failed. An object could not be found. Unable to directly book a resource for this meeting.”

This doesn’t happen with everyone, just about half the users. Any reason why? and can it be fixed without deleting the mailbox?