This error look like the page fetching errors. You might be facing the problem when users view / refresh or open a scheduled instance intermittently.
Ask your users to clean up the browser history. Check whether the same issue occurs from different browser.
You have confirmed that the issue is intermittent, so can you please provide a workflow. For e.g.
1. User x getting error; user y does not get any error. User x logs off, logs in back and no error found.
2. User x getting error; user y does not get any error. User x logs off, Shuts down the computer and login to the system no problem.
3. X and Y get same error for same reports simultaneously all other reports can be opened
Any others?