I have removed the 100-400L lens from the compatibility list (even though it came directly from the Canon website).
CHG_CANON wrote:
2x on 100-400L - No
As of Jan 2018 a Canon EF 2x extender cannot be used with any EOS body and an EF 100-400L I or II lens and retain Auto Focus.
Why?
The EF 100-400L zoom models have variable maximum aperture values at different focal lengths. At 100mm for the mark II lens, the maximum aperture is f/4.5. With a 2x converter, the maximum aperture is f/9. See comment above about all EOS bodies.