Hello,
The reason why this could be happening probably is that there could be some mails that are corrupt in the mailbox and to get this issue fixed you can try deleting the corrupt mails and then try taking a test backup.
This error could also occur if the mailbox being backed up is hidden in the Global Address List.
The following document can be referred for more information:
http://support.veritas.com/docs/243328
Stop all non essential third party software and Antivirus software services prior to backup.
Thanks,
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