Koen, did you figure this out? I have the EXACT same problem and your video confirmed it. The mobo as completely fine until I introduced a second monitor and then poof, it wouldn't boot after that. Prior to this the little voltage meter would read out the number beautifully and the internal power button actually worked. I've already tried everything presented on your post including switching out the PSU, no luck yet. If you solved it without replacing the mobo please let me know. Any response is much appreciated. Thanks.