Continuing the discussion from Youtube glitching on Mobile:
TL;DR: Try clearing browser history, cookies, and cached files. It got YouTube working for me again…for now, at least.
Had to do this because I got the “New users are limited to 3 replies” thing.
I might have found the solution. At least, it seems to be working for me. While messing around with Brave, I noticed that as soon as I blocked all cookies on YouTube, videos played just fine, though obviously it signed me out with no way to sign back in. But when I switched them back on (be it blocking cross-site ones, which I normally do, or allowing all) videos were broken again. So I decided to clear my browser history, cookies, and cached images and files. After signing back into YouTube, videos ran perfectly again. I’ll return to this if the problem pops up again, but for now this seems to be the best solution.