I am seeing this again with another client on 8.4. It occurs intermittently while a user is importing or exporting anything preventing them from completing the operation.

Today they had tried multiple times to export over a period of 30 minutes until it eventually worked. I checked the logs and though there were reports run and invoices/other documents printed during this time frame, I didn't see any long running operations that lasted for more than a few seconds.

In the client log I do see:
"M4status try again" around the times they are getting this error message, could this be related?