Hey again
Okay, it now works for me, and I want to share the solution in the hope that it will help someone else 🙂
I followed this advice:
A short video URL has the format: https://www.youtube.com/shorts/<Your VIDEO_ID>
But normal YouTube videos have the URL: https://www.youtube.com/watch?v=<Your VIDEO_ID>
It seems Squarespace can't locate the video if the link has a /shorts/ in the URL. Maybe Squarespace is programmed to be compatible only with a "watch?=v" URL. Therefore, you do this:
1) copy the YouTube web address URL
Important: Do not right-click on the video to "copy URL" or copy URL through the "share" function. These two methods add a "?feature=share" and a "si=1O7hxUt1usAD_py2" to the URL respectively, which Squarespace can't use.
2) Paste the YouTube web address URL into a new browser and change the URL into:
https://www.youtube.com/watch?v=<Your VIDEO_ID>
I paste it into a new browser window because it is a little more convenient than changing the URL inside Squarespace's menu.
3) copy this modified URL into Squarespace, and it should work