R12: Programs complete in Warning due to "Output Post-processor is Running But Has Not Picked up This Request"

Article "R12: Various Programs End With Warning: The Output Post-processor is Running But Has Not Picked up This Request" in My Oracle Support refers to R12.0.6 in document ID 1224684.1 BUT has a lot of information about the setup of the Concurrent Manager and Output Post Processor so it is a good one to read up on even if you know about this and have patched to help prevent the issue from happening!