Nothing on Youtube works any longer. It worked yesterday. Had to use a different browser.
Disabling and then reenabling the script seemed to work, though this is very disconcerting for me.
It may have been a coincidence. It stopped working again. Tried the same video on a browser that didn't have ViewTube on it. Also didn't work. Tried another video on that browser that didn't have ViewTube on it. Also didn't work. Tried the first video on my phone. It worked. Tried it on FireFox with ViewTube. Also worked. Likely cause: server error on YouTube's end.