SYMPTOMS
When you use the calendar, you may notice that calendar
items have been deleted or that calendar items have
disappeared from calendar folders. The following event ID
message may be logged in the Application event log:
Event Type: Error
Event Source: EXCDO
Event Category: General
Event ID: 8206
Computer: EXCHANGE
Description: Calendaring agent failed with error code
0x8000ffff while saving appointment.
You may notice that calendar items are deleted either on
one occasion each week, or according to some other schedule.
CAUSE
This behavior can occur if a file level virus scanner is
scanning the installable file system (IFS) M drive of the
Exchange 2000 computer, and may occur during a weekly or
daily scan of the server. The virus scanner may be set to
include all drives of the server. When the M drive is
scanned, the items in the information store are treated as
files. In this case, the virus scanner modifies a property
or attribute of the calendar items, and as a result
calendar items may be hidden.
RESOLUTION
To resolve this behavior, stop any virus scanner from
scanning the M drive. To recover lost calendar entries,
either restore calendar items from a backup that was made
before the calendar items were lost, or manually re-create
the calendar items.
found at
http://support.microsoft.com/default.aspx?scid=kb;en-us;299046