Here’s an unlisted video for comparison:
It’s possible that YouTube may be blocking requests from the IP of your server?
If you run, for example:
puts Onebox.preview('https://youtu.be/E5oGcdqz5xk')
in your server’s rails console, do you get something like:
<div class="onebox lazyYT lazyYT-container" data-youtube-id="E5oGcdqz5xk" data-youtube-title="demo unlisted link" data-parameters="feature=oembed&wmode=opaque">
<a href="https://www.youtube.com/watch?v=E5oGcdqz5xk" target="_blank" rel="nofollow ugc noopener">

</a>
</div>