From your description, it seems like a classic case of your server’s IP being throttled by YouTube, it was working, then YouTube felt something suspicious (possibly an AI false positive) then your IP was rate limited for some time, this is when you were unable to get any embeds to work. Maybe they released the IP after a review or the ban period expired when you were able to get it working again.
I don’t have enough data to prove this but there are reports floating around here about similar rate limiting from other social platforms due to the AI surge or other reasons.