Was just wondering if this was a bug or if my past luddite rants on autonomous driving have caused me to no longer be able to comment on videos?
It sounds as if this is an occurrence of the common bug that means you cannot comment on a video. It happens to lots of people, personally I've been experiencing it every few weeks for over a year, and it just seems to go away on its own after a while.
More mysteriously broken site infrastructure, in other words.
Oh, btw. I just had it happen again. If you are trying to reply to someone in the message, removing the @-reply from the post will normally let you post.