I've been having a problem recently with the R6. On occasion, I press the shutter button to take a photo and the shutter fails to work. It has failed when using AF and also using MF. It's failed with my Canon 100-400mm and with my Tamron wide-angle. Turning the camera off and back on usually fixes the problem for a while but the problem eventually recurs. Removing the battery and reinserting them works temporarily.
I'm using the current firmware. In fact, I upgraded to that firmware a week or so before the problem started. Canon customer service suggested I clear all the custom functions. Problem with that is I want to use those functions.
kvbarkley's suggestion is about all you can do. I suspect that Canon may not take the camera for repair until you do what they suggested. Even if they do, they will reset it themselves, so you'll loose those settings anyway.
I know it's a PIA, but that is what I would do. I have the R6 and have not had that problem.