This is happening to me too. I can make new comments but I can't reply to existing ones. I get messages saying "This value is not a valid captcha", but it's not offering me captchas in the first place.
Considering the captcha used is some of the worst kind imaginable (whenever you click on an image, another one takes its place, like Orcs killed in the Siege of Minas Tirith), I doubt the site owners care about our issues. What a joke.