Backup exec cannot extract mailbox messages from the Exchange backup

Hi

Does anyone know why my backup jobs randomly fails with this error code, it happens once every 2 weeks or so

My backup exec is fully patched and running Backup Exec 2012

Backup- \Microsoft Information Store\Mailbox Database 2012070313V-79-57344-759 - Unable to complete the operation for the following reason: VFF Open Failure.  This can be caused by low memory or disk resources.

Final error: 0xe00002f7 - Cannot extract mailbox messages from the Exchange backup. Review the job log for more information.