This unfortunately has happened to me multiple times, and it is a bit of an annoyance to switch to one of your "back up" browsers. I found that at one point i updated everything safari related and the OS updates (yes including flash), and had to wait four weeks until YouTube functioned properly. It's unfortunately one of those things that you have to let work itself out. There seems to be one temporary fix which is doing the following: Go to safari > preference > privacy > remove all website data...If this only works for a short amount of time, just use Firefox or Chrome for YouTube, I hate to say it but that's your best bet