I was receiving the above error usually immediately after netbackup attempted consolidating the backup.
My particular issue:
Seemed to revolve around the fact that I had "Storage I/O" control enabled on my datastores that was causing this issue. It was odd that only 1 VM seemed to be affected, but after turning it off the symptoms stopped. Reading the KB above seems to allude to this fact, but I'm still researching.
Turning off Storage I/O control.