I just ran into this " OIB already exists" issue while evaluating V6.
I know the cause in my case, as the log from the first attempt to run this job showed a failed attempt to lookup my vSphere Server's ip address.
For some reason that left the VM with data in the backup set yet I kept getting the "OIB" error.
As this is just an evaluation I did end up deleting the backup data for this one VM and a re-run of the job was successful.
A good question for the QA\ENG guys would be why this simple error caused a failed backup? I mean it doesn't look it ever tried to lookup the VS Server address again. It seems there was a maint window on one of the internal DNS servers and the other one was not configured correctly that caused the DNS lookup issue. These issues have been addressed already.
I just wanted to add my experience with this error to the list of possible causes.