Wondering if you got anything resolved with this? I have a similar issue. Just got a replacement back due to a failed IMU and now I have extremely dark image no matter if its video or still unless I put it into an auto mode, hyperlight or HDR. I don't have the flashing that you did but it has, for a few seconds at a time, allowed light in as expected. I've tried multiple known good cables and mobile devices. Refreshed latest firmware (1.90.0670), downgraded 1 vs, and upgraded again, checking image each step of the way, all to no avail. I do run in manual so that i can control the exposure but on the original had no issues with my settings (ISO100, F5-8, S 1/50-1/60) which are the same settings I use on my M2Zoom. Another strange thing is that with regards to the 5D button, I have it prog to incr/decr EV for L to R and it changes the shutter speed instead. I don't want to send this thing back.....again, but don't see any way around it.
These screen shots are in full lighting indoors. There's no reason there is not enough light for this size sensor, I take videos and time lapses of sunsets and sunrises and have gotten much better video/stills with 1/4 of the lighting on the original
M2P.