Exchange 2003 Problem

We recently upgraded to Exchange 2003, We have a problem slowing showing up. Some of our users are reporting a problem where a reoccuring appointment in there calendar has gone corrupted. When the appointment is clicked on a messages pops up “Cannot read an instance of this recurring appointment. Close any open appointments and try again or recreate appointment” You must close outlook and reopen it to get rid of this message. The only solution so far has been to restore mailboxes from tape prior to before the problem started to occur. I intially thought that the problem was caused by the upgrade but it was not until a week and a half later that this started. Any help would be greatly appreciated and time saving. Thank you