I think rebooting might have worked for me. I had to do something several times. My memory's not so good. I suggest signing out of youtube, rebooting. It should work. I had the same trouble several times lately with youtube videos of any sort.
I finished watching the whole thing last night. I was very impressed by the information she has put together. The quality or style of the documentary isn't the best, but like I say, she obviously has done a lot of research and synthesized the data so it's really interesting. I guess from what I know of some of the knowledge that many posters here have that they've watched it before. For those who haven't, it's well worth the 5 hours of time it takes.