Single instance backup enables Backup Exec (tm) to only have to back up one copy of an attachment that has been sent to multiple user's mailboxes. Attachments over a certain size are backed up as pointers and then the actual attachment is backed up at the end of the job. Single instance backup allows for smaller backups because similar attachments are only backed up once.

The primary reason that the error occurs is because Enable single instance backup for message attachments is selected in the Backup Job Properties and an attachment has been moved or deleted in Exchange during the backup (anti-virus quarantine, deletion, improper scanning, etc).

To prevent the error from occurring, make sure that virus scans are not being run on the following files at any time:

If the error still occurs after performing the steps above, either disable virus scanning during mailbox backups or clear the selection of the single instance option in Job Properties | Settings | Exchange (Figure 1).

Figure 1