Ok, so I started having the same problem about one week ago (on firefox aswell) and I managed to find out what causes it, at least in my case.
So here is what happens, if you are logged on two google accounts and you use the secondary account for youtube, the embeds will stop working.
You have to either go to Youtube.com and switch to the primary account OR logoff both gmail accounts and make sure you log in first on the account you are planning to use on youtube.
More people on reddit are reporting the same problem: https://www.reddit.com/r/youtube/comments/kx19gb/youtube_embeds_not_working_if_logged_into_youtube/