Bad news, @TeamYouTube, I can no longer see the video thumbnails - I'm seeing just plain-white thumbnails with an icon that I do not know of.
Tried everything I could, and it never worked, still showing white thumbnails.
I'd show you, but I'm on an Xbox One S console.
Hmm, that's strange. What troubleshooting steps have you tried so far? If you haven't already, try reinstalling the app and see if it does the trick. Also, make sure that your firmware is up-to-date. Keep us posted!