Turns out commenting has been broken for quite a while, with mainly only spammers being able to post 😉
I’ve ‘fixed’ this with a really, really ugly hack, but at least you should be able to comment now. Phew!

EDIT: It appears they are not fully working, only for logged in users, we’ll investigate.

EDIT EDIT: Really, truly, comments should be running fine now. But please let us know if you experience problems.