This is not a Microsoft Problem but a Veritas Problem.
We opened up a call with microsoft who showed us log infromation showing that backupexec was causing VSS errors.
We are still (3 months on) waiting for Veritas to come back to us with a solution. this only occures on certain storage groups.
as a work around you can create a new storage group and move all mailbox's to that storage group till that one fails and so on.
other wise there is always net backup :(