I have been unable to watch YouTube videos for a couple of months. There were some people on the Google Forums reporting the same problem, also posting screenshots, but no one seemed to be able to fix it.
Finally, checking with despair my Chrome Dev Tools, I found the problem! In my rush to block ads and avoid the slowing down produced by all the requests to external domains, I ended up redirecting s.ytimg.com to my own PC. Sadly, it seems YouTube is taking some CSSes and other stuff from there, reason why I was unable to watch any video and load the home page with a decent view.
Simply get rid of that row in your hosts file (or wherever you may have blocked it), save, and you are done!