Embeddd Youtube videos do not work

I have quite a few embedded videos on my website right now, and it seems most of them do not work anymore. The <iframe> code is still there, but nothing shows up after publishing the post/page. I tries switching to another theme and the videos are there, working properly.

The thing is I have no idea where to start searching for a solution, so this is why I am posting the problem here, to see if anyone else has/had the same issue.