This morning (January 19), I attempted to post some YouTube videos and, while they look ok in the Preview Pane, error when the post is added.
Here’s a link to a post from last week that works and right underneath it is a post made today.
Is this a YouTube problem?
Or did you guys do something to me? 
Having the same problem. I thought Onebox had updated but can’t find any new documentation for it.
I can easily reproduce the problem. Looking into it. The onebox works when editing, but once you submit the post it’s broken with youtube.com/watch?v=undefined as the url.
I will get it fixed tonight 
This now appears to be fixed.
Thank you!
@neil fixed this via:
https://github.com/discourse/onebox/commit/e9f04930c0c17b47c31b7a15df4deaa243515f04