I have a new (to me) 1DX that I've been using with my 28-135. My 28-300 is normally on my 1D. On my last shoot I took my 28-300 off the 1D and put it on the 1DX. I got the Error 1 message about every 5-10 clicks of the shutter! I found that turning the camera off and back on corrected it, but then it would happen again. The camera works fine with all my other lenses. And the lens works fine with all my other cameras. I don't mean to hijack your thread, but to simply quote a similar occurrence.