Shared Calendars list (Other Calendars) disappears
 

In Outlook 2003, When shared calendars are opened, they remain in a list on
the navigation bar (Other Calendar). It is convenient to leave commonly
used calendars displaying in this list.
Periodically for no reason we know of, they disappear and the shared
calendars must be opened one by one again.

What causes them to disappear and how can that be prevented?
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Elaine Clark
End User Technician