@andrew84 That could be. It could explain why the problem I had literally came on overnight, and maybe even why it was so weirdly specific. (Videos would play unless I was trying to play them in a youtube.com tab that saw me as signed in.) It could also explain why they are working again, as of now. (I'm 6 hours behind CEST, if that matters.)
Anyway, "try to update the adblock lists" and/or "wait for an update to the adblock lists" may be something to put on the list of things to try when something like this happens.