What I’m trying to do:
Regular monthly task, however, I got this error 
:
anvil.server.RuntimeUnavailableError: Data payload too big - please use Media objects to transfer large amounts of data.
Session ID: 7XBHIYIEUH5RUYI42BOPVVFBBLG7Y47S
At the beginning I thought it has to do with using “q.fetch_only”, however, after removing the “q.fetch_only” the problem persist 
Also, the session log doesn’t pin point the code line which makes it very hard to track 
When used, q.fetch_only tends to reduce the amount of data transferred. So I’m puzzled by your line of reasoning.
The volunteers on this forum do not have access to your Sessions, nor your app, so we’ll need more information, specifically, about what your program was trying to accomplish at the time, and the circumstances, before we can be of much help.
In the meantime, I suggest that you read that error message carefully, and take seriously the suggestion it is making.