I get
Whoops!
You need to enable javascript to do that.
but I checked and javascript seems to be enabled in my browser.
Edit: Oh. A different post did work. But the original one still doesn't. I assumed it was consistent because it keeps happening for one particular post.
Posts
If it was on every post you tried to agree with, I'd think it was something with your machine/browser. But if it's just giving you an error on that one post, it might be something that needs investigated.
Still won't let me agree. And other posts do let me.