calendar sharing problem in outlook 2003
 

I have a Windows 2003 SBS running using Exchange server 2003. All clients
are using Office 2003 and are sharing calendars. One of the PC's now will
not allow others to add or make changes to his calendar. He can make and
add changes but others cannot. They get the error "the item could not be
saved to this folder. the folder has been deleted or moved, or you do not
have permission. do you want to save a copy of it in the default folder for
the item?" I have already archived his email to reduce the space he uses.
Again, he can make changes but the other staff cannot. I've unshared his
calendar, I've changed permissions, I've compared his settings with
others....no luck!!. Please help!!