I have several playlist iframe players on my site that are currently all broken. The iframe shows a preview like the video is unavailable, and pressing play results in a "Video unavailable" error message.
I'm seeing the same behavior on youtube.com when grabbing the embed html code from the share prompt. Try this on any playlist page: click the share button on the left side of the page, then select embed. The iframe player will look like this (i.e. the same as the iframe on my site).
I've been facing the same issue where my playlist embeds stopped working, same thing for the generated youtube embed URL.
Apparently youtube broke something in their playlist embed mechanism, I've tried reaching out to the support but no response as of now.
There still seems to be a way to get things working for now, I've managed to embed a playlist by removing the "videoseries" part from the URL, so by using https://www.youtube.com/embed/?list=[PLAYLIST_ID] it worked for me.
"videoseries" was breaking URLs on April 14&15, 2021. Today April 16 YouTube appears to have fixed that problem. See What would cause this youtube embed string to stop working?
