Before getting insight on two common issue , read this post on "Output Post Processor – OPP ^[1]"

I S S U E # 1: This error often you get.

The Output Post-processor is running but has not picked up this request. No further attempts will be made to post-process this request, and the request will be marked with Warning status.

Action

• You need to set the Profile Option "Concurrent: OPP Response Timeout" to a higher value may be necessary.
• If the value of the Profile Option „Concurrent:OPP Response Timeout. is 120 then increase it to 240. (Value * 2).
• Increase the number of processes or threads (or both) .

I S S U E # 2 :Program end-up with this error.

The Concurrent Manager has timed out waiting for the Output Post Processor to finish this Request.
Check that there are enough Output Post Processor Service Processes running. More information may be found in the Service Process logfile.

Action

• You need to check the Profile Option "Concurrent:OPP Process Timeout".
• Increase the setting to a higher value.

For example: Set the value to 10800 sec (3 hours). After this change, the Concurrent Request will wait maximum 3 hours, for the Output Post Processor to finish the Request.

hope this helps.