For many months I've been getting that a lot. And not getting it. Sometimes embedding a video works and sometimes it doesn't, but if it works then it always works for that particular video and if it doesn't work the first time then it won't ever work (in my experience).
My practice has evolved to trying to embed the video but also providing the URL link in case that doesn't work. Furthermore, I will use Preview as the first test: if the thumbnail preview image is present, then it should work, but if the background is completely black then it won't.
I suspect that the issue is on the YouTube side and not the EvC side.
Using the "youtu.be" form of the link seems to be the best supported method these days.
I don't understand. Why would we need to route through Belgium (.be)? Do their proxies do something different?
ASIDE: There seems to be something in British humor about Belgium that I don't quite get. For example, in Hitchhiker's Guide to the Galaxy, Zaphod Beeblebrox's curse word would be "Belgium!" (used at least once; last I listened was four decades ago -- I recorded it from the local NPR station when it aired and listened to it while driving home from North Dakota to So.Calif.).
And then on Doctor Who ("Time Crash", a special where the 10th Doctor meets the Fifth Doctor, who in real life is David Tennent's father-in-law), I seem to recall something about a temporal anomaly blowing a hole in the space-time continuum "about half the size of Belgium?".
Now to experiment with that YouTube link, https://www.youtube.com/watch?v=x-J7XB3OBHE -- it's reportedly the only decent copy of "Time Crash" left on YouTube, but it's done with a voice-over in Russian that obliterates much of the original audio (same happened to their copy of the German film, Viktor und Viktoria (1933), which was remade as "Victor/Victoria" ).
Information from an external source - Message 1 and Message 7 YouTube clips do not work for me. -YouTube "play" button appears with a black background in the clip area -clicking on the play button gives an error
The Bugs Bunny one in Message 2 works fine. -YouTube "play" button appears on a video background (I can tell the video will be about bugs bunny before even clicking on it from the still image in the background) -clicking on the play button play the video just fine
And as I report, the same formatting produces some videos that work fine and others that don't work at all (as you just described).
For example, the embedding of a video in my Message 195 worked just find using utube and youtube (no Belgium!), yet other attempts of mine also using those keywords failed (sorry, but no time to hunt them down right now).
Again, I suspect some setting or property of the individual video to be at play.