Due to GDPR, websites are required to use privacy enhanced embedding for YouTube videos. The extension currently doesn't seem able to parse the privacy enhanced urls which are hosted on a different urls (youtube-nocookie dot com/embed/[YouTubeVideoID]) instead https://www.youtube.com/watch?v=[YouTubeVideoID].
This is what the iframe embed code from YouTube looks like <iframe width="560" height="315" src="https://www.youtube-nocookie dot com/embed/[YouTubeVideoID]" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
(Please note: due to spam filter here I had to change .com to dot com in the URls)