YouTube white screen, black box in the corner

When on YouTube, on Safari, I go into fullscreen, and the screen is usually white, with a black box in the corner, and I have to end up refreshing. I am on a MacBook Pro Retina 2014 model. I have tried uninstalling my flash player, and reinstalling it, but it did nothing. I also tried to do the "run on 32-bit Safari" method, but I don't see the option to. Please help me!