Here is some more info for you.
Here are the forms of the links of YouTube that are playlists:
https://youtu.be/4PQrz8F0dBI?list=PLHNAKnd_6CWfOT0MUx5rqhlz247K-_KSJ
or
https://www.youtube.com/watch?v=4PQrz8F0dBI&feature=youtu.be&list=PLHNAKnd_6CWfOT0MUx5rqhlz247K-_KSJ
And WordPress Embeds the url as such:
https://www.youtube.com/embed/videoseries?list=PLHNAKnd_6CWcQH5Vurnzuf15PRQ2-WxSL
Your plugin tries to grab the Video ID and then embeds the playlist as the usual embed, but with no ID.. so you end up with an invalid video embed.
Since the Playlists always have a list= in the URL, worst case, you could perhaps just disable your URL rewrite for preg_match (anything with a ‘list=’ in the URL.)
Or you could do the URL rewrite with ‘/embed/videoseries?list=‘ and that long number at the end.
Thanks!
Charles