In the end, what helped us to complete the report successfully was the below:
Increased the value of the Concurrent:OPP Timeout profile option(Concurrent:OPP Process Timeout) to 10800 seconds from 600 seconds
600 seconds (10 min) should be more than enough for this setting - The fact that you had to increase this value is indicative of other issues in your environment - possibly too little CPU/RAM
Glad you have sorted this - note that the fnd_concurrent_requests table has the fields pp_Start_date and pp_end_date which will show how long post processing is taking.
Good to a good idea to monitor this in relation to the setting you have made in the profile option Concurrent:OPP Timeout profile option though 3 hours should cover most things!!!!