Can't change Outlook Calendar Appointments

[leeym]

Our office runs Microsoft SBS 2003 and all of our workstations are using Outlook 2003. For some reason, some of our users cannot change their appointments in Outlook Calendar. This is the error message they get:

Your changes could not be saved because you don't have permission to modify some or all of the items in the folder. Do you want to save a copy of this item in the default for the item?

Any ideas on how to overcome this?

Thanks