Full -- The job failed with the following error: Cannot extract mailbox messages from the Exchange backup. Review the job log for more information.

Hi

Backup Exec 2010 R3 on Windows Server 2008 R2 Std.

Completely randomly and intermittently, I get an error of

Full -- The job failed with the following error: Cannot extract mailbox messages from the Exchange backup. Review the job log for more information.

I have seen several solutions which I have tried previously, mainly disabling GRT, then trying again, but this happens from day to day, regardless of if anything is done on the server (i.e no reboots, noone even logs on yet next backup is fine)

Any ideas?