JetPack throwing CAPTCHA error when attempting to post a comment

I checked and the error is coming from the iFrame located on https://jetpack.wordpress.com/jetpack-comments/?blogid=…

Further information, I’m getting HTTP 500 errors when a user attempts to post a comment:

[18/Jun/2013:16:53:17 +0000] “POST /wp-comments-post.php?for=jetpack HTTP/1.1” 500 4184 “https://jetpack.wordpress.com/jetpack-comment/” “Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537
.36 (KHTML, like Gecko) Chrome/27.0.1453.110 Safari/537.36″

Do you use any security plugin, Anti Spam plugins, or other comment plugins on your site? A few plugins are known to conflict with Jetpack Comments.

If that doesn’t help, could you send me a list of all active plugins on your site?

Thanks!