I think I've figured it out. YouTube are forcing https, and our embedding plugin doesn't support https.
Well then. There's nothing we can do. The trick of embedding the video by deleting the "s" in "https" is gone.
We'll have to take a snapshot of the cover of the YouTube video, place it on an image server and post the image under the link.
Basically that's what I do for Twitter (Twitter is my image server) because Twitter posts with a YouTube video link unless clicked on, will not show the embedded video, but a link tag (until you click on the post). Sadly, many years ago I noticed that Twitter posts which automatically showed a picture on the time-line were 10 times more likely (using a counter) to bring customers to my shop compared to posts that only had a link and words. People don't like clicking links without a picture of where they're being directed. Whether or not Twitter will show an embedded picture or video, depends on an agreement between the host and Twitter, so certain platforms can freely embed pictures and videos and use space on Twitter servers, while others (eg Instagram, ) just don't.