In case you missed it (like I did), Canon has recently released a minor firmware update for the EOS-1D X Mark II.
Firmware Version 1.1.6 incorporate the following fix:
- Fixes a phenomenon in which the camera may not function normally when images are simultaneously displayed on the camera’s LCD monitor and an external monitor (via HDMI output).
It is always worth downloading and installing the latest firmware even if you feel the listed fixes are not relevant to your photography. Manufacturers often make other undocumented improvements in firmware updates.