Ray uses a Chromebook and after a recent update, it won't play videos on YouTube. What happened? Leo says it sounds like the update didn't really install properly. A "power wash" of the OS may fix the problem. There could also be an ad blocker that's preventing the video from playing. Ray should also turn off hardware acceleration in the settings.
It may be an issue on YouTube's end. There may be an HTML5 problem. He should remove all of the plugins and reset the browser. He can also go to YouTube.com/html5 and make sure it's not misinterpreting what his device is. He can also change the codecs it'll support. He can uncheck some and see if it works better.
It's definitely a codec issue. An update will likely come soon to fix it.