Why can't I post comments on youtube?

I’m not talking about videos that won’t allow comments (obviously). I’m talking about videos with comments, and even my own videos. I type in my message in the box provided, but the character count (you’re only allowed 500 chars) doesn’t go down, and when I hit “post” an error message comes up.

This happens even when trying to comment on my own videos.

I am logged in normally, and can still send messages, post new videos etc. Is this one of them cookie things?