Yikes, the buffering is real:
07:03:23.731: adding 23 milliseconds of audio buffering, total audio buffering is now 743 milliseconds
07:05:46.445: Max audio buffering reached!
07:05:46.445: adding 301 milliseconds of audio buffering, total audio buffering is now 1044 milliseconds
For now I just deleted the media source which was a looping MP4 file at about 226MB. Never had an issue before though. This is a fresh install of Windows 10 if it makes any difference, installed about a week ago.
Tried various combinations of tick boxes for the looping vid in OBS, Still crashes if I switch scenes to it after a bit, and sometimes even if I don't.