An interesting finding after a weekend of troubleshooting.
This can maybe be something you want to have a look into @Falco or @codinghorror?
At the moment, reading the source code of youtube_onebox.rb , I see it supports 3 URLs from which the video id is extracted.
http://youtu.be/<videoid>https://www.youtube.com/embed/<videoid>https://www.youtube.com/watch?v=<videoid>
Every attempt to onebox videos whose links are in 1. and 3. formats fail with onebox returning 404 (which I think is related to our IP being banned)
When I try to embed links having format as 2., it works!
I wonder if and how this could be related to the fact explained in this post.
Some understanding (and logging) of the inner working of onebox (i.e. what exact call(s) is(are) made to youtube) would be super useful…