In our case it was because of a username/password containing special characters, we solved by using only alphanummeric characters. the bpresolver log on the frontend might give an indication
Just to let you know, today Symantec released a technote on document level backups/restores in 6.5.3. Hopefully it's useful to someone. I will be trying it as soon as I have my hands on 6.5.3.
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