It appears to be broken on our sites. Generating embed codes for any playlist shows an error for us. To get it working on my site, I had to replace "videoseries" in the URL with "playlist" (no quotes) and then it worked. Really strange issue.
Yes it's broken, but then it's not.
You can literally change 'videoseries' to anything else and it will work. in fact, you can have it like this and still works:
https://www.youtube.com/embed/?list=YOUR-PLAYLIST-ID