Lost calendar permissions

We are currently working in a Windows 2003 domain with Exchange 2003, and all
clients have either 2000/XP with Office/Outlook 2003. I have a user that is
granting several users permission as reviewer for her calendar. She is
setting this both at the calendar as well as the mailbox level. One of the
users she grants permission to, who is in the same domain, is able to see the
calendar fine when she first grants permission. When she logs off and comes
in the next day she gets and error when attempting to open the calendar
stating that "Unable to display the folder. The calendar folder can't be
found". The permissions on the calendar owner are not being changed, she has
to toggle her permission off and back to reviewer in order to give her access
again. None of the other users being granted reviewer permissions are having
this problem. This has been happening for several weeks now, can anyone help?
--
Devin