Whoa, IT WORKS.
Just minutes after posting my message above, I deleted the previous video block and inserted a new one, then pasted in the YouTube video URL as always and SUCCESS. I also found an email response from Squarespace customer services:
Thank you for reporting this!
The trouble you were experiencing with adding video has been resolved, so you should be able to add videos to your site without issues this time. Apologies for any inconvenience this m