While this is my first post to this forum, I have been reading and learning from it for a while... so I thought it was time to try giving something back
Yesterday for the first time, when I tried to use my Canon 70D with a kenko DGX (blue dot version) 2x extender and the Canon 400 5.6L, the camera basically froze, refused to turn off or write pictures to card. The only way to resume it was to remove the battery.
The same problem (or some variations of it) kept reappearing every time I try that combo. The teleconverter works fine with other lenses, and the lens works fine with other teleconverters. At first I could not figure out what was wrong... also because I successfully used that combo before!
Then I remembered I read something similar here regarding the 5D mark III:http://www.canonrumors.com/forum/index.php?topic=4977.0
I thus realized that the reason for the new behavior was that I recently tried to AF microadjustment that lens (luckily, no need for that, or maybe needed just -1 in some situations).
I then switched off the AF microadjustements, and everything worked flawlessly again. By the way, the new Dual pixel AF works quite nicely with that combo, allowing AF at 800mm/F11 (sometimes it can hunt a bit, but it is fast enough to take pics of a perched bird)! Too bad you can't microadjust...
Hope this helps!