Maybe it's something similar to what I experience with my 60D and ML2.3?
Having Headphone mon. switched on and the dslr cord attached to a pair of headphones and rebooting the camera the screen is black and no audio is heard.
However if I keep the dslr cable in the camera and remove the headphone cord and reboot the camera and then attach the headphone cord to the dslr cord both the screen and the audio works.
So maybe there is some sort of bug when rebooting the camera with the headphones attached?
Having Headphone mon. switched on and the dslr cord attached to a pair of headphones and rebooting the camera the screen is black and no audio is heard.
However if I keep the dslr cable in the camera and remove the headphone cord and reboot the camera and then attach the headphone cord to the dslr cord both the screen and the audio works.
So maybe there is some sort of bug when rebooting the camera with the headphones attached?