Hi, I'm having similar problems, but not at random times, only when I wake the NUC from sleep. And the HDMI signal is also going though an AV receiver, which may or may not be on (passthrough or not). Most of the time, it happens when I wake the NUC from sleep, and only the TV is on (while the receiver was still on when the NUC went to sleep). The NUC does not seem to like to switch HDMI outputs apparently. But lately, it happens more often, almost every time I wake the NUC from sleep, and its annoying. Only solution is a hard reboot. That was my $0.02.
One question I have is what is STRG in " STRG-ALT-F4". I dont know this shortcut, and it sounds very interesting?!
Well, this doesn't sound too much like my problem.
Anyway, "Ctrl+Alt+F4" is the default hotkey combination of the Intel graphics driver to "activate digital display". Indeed it also switches between the 1st and 2nd HDMI output.
Dumb of me to have written "STRG" instead of "CTRL" (the "STRG" key is the equivalent for "CTRL" on a German keyboard).
As a side note, I could observe the HDMI drop with a fullscreen 24p application (with some animation) in the meantime. So it's not an issue of static contents as I originally assumed. Still hope someone from Intel will look into this...
For the protocol: today my Denon AVR 4311 wanted to upgrade its firmware so I let it do that. Indeed this was the first update I ever installed since the receiver was pretty mature as I bought it and in the meantime it's considered an "obsolete" product. So I was kinda surprised that there was an update and kinda hoped this could maybe fix the HDMI issues. Unfortunately it didn't. The behavior is still exactly as described above.