Having a very strange issue with my Canon 6d.
This issue began happening about 1 month ago. I typically shoot in Av mode and first noticed the problem here. When in Av, the "menu" and "image review" buttons are unresponsive.
I thought at first that perhaps there was a hardware issue with the buttons. However, I switched to automatic mode and was able to access the menu and the image review.... weird... So after trying all of the modes, here is what I see.
Symptom: When in AV/Tv/P/M/B/C1/C2 the Menu and Image review buttons are unresponsive. When in SCN/CA/A the camera functions normally and Menu / Image review are accessible.
Troubleshooting: Upgrading the firmware (was on 1.3 and upgraded to 1.4). Cleaning the camera. Drying the camera out using a desicant (just in case). Switching lenses. Powering the camera on with no lens. Using a different battery. Using a different SD card. - All with no effect.
Unfortunately, in the automatic modes I am unable to reset all hardware settings. Also, the canon 6d's bios battery is not accessible (damnit) - so a hard reset is not an option without disassembling the camera.
Any ideas? I'm stumped here.
I would call Canon support. I think the internal "battery" is actually very low capacity. It occurs to me that leaving the main battery out of the camera for a few days may drain the internal battery enough to reset the camera.
What's odd about your issue is that while many of the buttons on the back of the camera can be re-assigned to different functions... I don't think the 'menu' button is one of them (I think that button can only ever be the 'menu' button and I don't think there's any setting to disable it). So I can't imagine what setting change could cause this.
Also, the canon 6d's bios battery is not accessible (damnit) - so a hard reset is not an option without disassembling the camera.
For cameras with non-accessible backup battery Canon recommend that you remove the main battery, turn the camera on and hold the shutter button for 30 seconds. Not sure where I read this and I have never done it but it has to be worth a try.