Problem with updating reoccuring meeting

Background: There's a reoccuring meeting (Thursdays at 10:30AM)) sent to 3
people. The organizer updates the reoccuring meeting time for this past week
from the normal 10:30AM to 1:30PM. 2 of the users have no problems.

However, 1 user gets the error message "The operation failed. An object
could not be found" when she clicks the Accept button. This user also gets
the error message "Can't open this item. The operation failed. An object
could not be found" when she tries to open the meeting on her calendar.

Also, we use backup exec to backup the mailboxes and we see errors for the
user's calendar. We also see on the backup exec log that the meeting request
file is corrupted.

Can anyone offer any suggestions/solutions to this?

Thanks.