I tried on multiple forums I host, and here’s the result each time:
However, it works on try.discourse and meta:
Example playlist link: https://www.youtube.com/playlist?list=OLAK5uy_kzSjyMWjRrEdqpP9x8HOq1MFcrItGQFWw
I have no JS error message, Discourse 2.9.0.beta2, tried in safe mode, and there is nothing in the error logs.
So I guess it may be related to a site setting, but which one?
Maybe it’s because of the server location?
The solution in that topic works only for videos, not for playlists and channels. So for these oneboxing doesn’t work anymore.
You can try to delete the “www” like it is described here Youtube embeddings have stopped working for servers in Europe - #14 by Iceman
That worked for me using the example playlist above. 
Previous tests gave me the same broken image result, but removing the www worked a treat:
Does that help for channels too?
I’m not that hot on YouTube links, but copying a channel playlist seems to work fine with www (and the m ones for mobile). I may be grabbing the wrong links though. 
Have you got an example one I could try for consistency?
I cannot get that one to work (featured link or regular onebox).
I’m seeing the same, in that www.youtube links to playlists fail in their onebox whereas removing the www makes it work. My digitalocean server is in London. The same is true for my Hetzner server in Germany.
Both of the types of links onebox OK on this server (meta.discourse).
So it does look like a Europe thing.
https://www.youtube.com/playlist?list=PLWVQAe9fXZolICdfMjY4MNdYdMgTOx5QT
https://youtube.com/playlist?list=PLWVQAe9fXZolICdfMjY4MNdYdMgTOx5QT
