Yesterday I started having an issue with my Canon 1Dx Mark II and 400mm f/2.8 IS II - When the lens is attached to my camera, nearly all buttons stop working, including review, menu buttons, set, AF-On, etc. The only buttons/dials that work are the shutter button, aperture and shutter speed dials (back and top dials), and joystick. The camera works fine with other lenses, and the same probably happened when I connected the lens to three different cameras. If I turn on the camera without the lens then attach it, or detach and re-attach the lens while the camera is on, all the buttons work fine briefly, then stop working. I've tried gently brushing and using a lens blower on the lens contacts. I have set up service with Canon and will be able to ship off the lens in two days, but wanted to see if anyone on the forum had experience or solutions for this issue.
Solved! Go to Solution.
Try pressing and releasing each of the focus stop buttons located around the far end of the lens. These telephoto primes will cause the attached body to act that way when one of those buttons is depressed, I just tried it on my 1DX II / EF 400 f2.8 IS II and 1DX III / EF 800 f5.6 IS and get exactly the symptoms you described.
I wonder if one of your buttons has become stuck, if so cycling it a couple of times could clear the fault.