I installed Windows 8.1 2 days ago and I'm pretty sure that's when this issue started. About half of the youtube videos I try to watch are infinitely stuck at this screen:
It seems that rightclicking the videos that have this show it's the HTML5 player whereas those that work use the Flash player (at least: that's what the context menu indicates).
I have tried both working and not working videos in Internet Explorer and they both work there.
I'm using Google Chrome 32.0.1700.76 m. I used to use the Youtube Center plugin as well but enabling/disabling this makes no difference.
According to https://www.youtube.com/html5 everything HTML5 should be supported:
Right now it is set to use the standard player instead of the HTML5 one; changing it either way doesn't solve anything.
Any ideas as to what could be causing this?
Edit:
A friend verified that he has the same problem on his Windows 8.1 installation. I just tried it on my laptop as well (Windows 8) and the issue occurs there as well.
You can try it yourself with this video.