Please post the contents of the concurrent request log to better diagnose.
Please upload the log file.
You may need to check OPP for your instance.
Its size ,logfile and weather it is configured properly or not.
Please upload the request log and the Internal manager log when the issue happened?
Also is the issue happening to one report or multiple reports?
Is the issue specific to xml reports or rdf reports?
Thanks for the post.
Please upload the OPP/CP log for more details.