I started using my R5 in place of my 1Dx for shooting sports (football and waterpolo). I am only using the EVF, the backscreen is turned towards the camera. I'm shooting JPG only, no video. I am using Electronic Shutter H+. Last week shooting waterpolo on 2 separate days, the camera gave me the red flashing overheating message, and also the camera froze/locked up. What i mean by that is that the last image I took stayed in the EVF, and no buttons would change that, so I could not take any pictures. Turning camera off & on again did not unlock the camera (the image went away, but the EVF stayed black). Taking the battery out and putting them back in did seem to temporarily fix freezing problem, though the flashing overheating warning appeared again very quickly.
The next day I shot another waterpolo game, and the same thing happened so I took my camera to CPS for repair.
Canon CPS did quickly have my camera serviced, and I picked up the camera 2 days after dropping it off.
The notes on the service tag said "...the product performed according to specifications. Unable to duplicate issue. The main circuit board was replaced as a precaution."
The day after picking up the repaired R5 I was again shooting another waterpolo game. This time, no overheating warning, however twice during the game the camera froze, requiring me to take the batteries out in order to unfreeze the camera and continue shooting.
Something still not right with this camera, and I'm afraid the problem won't be fixed before my warranty runs out. I can understand that they can't duplicate the issue if it is only happening to me during waterpolo on warm pool decks. I've never had an issue with the R5 shooting football, however with football there is considerable down-time between plays, so I'm not looking through the EVF continuously. I have a feeling this issue may still be heat-related.
I'd be intereseted if anyone else is using the R5 for continuous action outdoor-sports (tennis, waterpolo) where there is little break from looking through the EVF. With football and baseball, I'm finding that the downtime between action must keep the camera temps at a range that does not cause the locking/freezing problem I'm getting with waterpolo. With waterpolo the EVF is basically active for 90% of the game-time.
Your issue almost sounds environment related. I know indoor swimming pools can often times be pretty humid, and WARM.
BTW, what lens(es) are you using?