Backup Exec 2010 - E0009B61 (Security Errors)

Hi all, having a bit of a problem with a scheduled nightly backup.

Almost every night, it errors with the following:

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This is very strange, as the backup fails 80% of the time, but sometimes it works, and when it works the backup is 100% successful.
I have tried using several different accounts, all with the same issue, below is the resource credential check, which passes.

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Thanks in advance for the help! 

Nick.