It seems to be a very recent (17/4/2014) issue between YouTube and HTTPS Everywhere. When HTTPS Everywhere is in action, all YouTube videos fail to load with YouTube replying that there's an error. So just disable HTTPS Everywhere for YouTube and it WILL be OK. Hope this issue gets resolved in the next week or so.
---------------------------------------(Update on 19/6/2014)--------------------------------------------
Seems that the issue has been resolved to date. I believe it was fixed quite a while ago but I was busy.
