I was having this same issue. My issue was caused by the “WP Captcha Free” plugin. When I disabled this plug-in, I was able to comment on a post.
When I first installed this plug-in, I tested and things worked fine, however, since then I have installed lots of other plugins, so I am thinking that something clashed.
I think its a good idea, if you see this error, leave a comment of what you found fixed it so others can see as well!
