I'm using IE6 here (I can't change government machine configuration, so no IE7 or FF here.) and cannot click "Reply" button occasionally too (About half of the times "the first reply after log-in" works, for the other time it don't. And seems once it works, it'll work for other pages too.)
To use it, I have to click that address bar and delete everything after the last slash("/"), then hit enter. Then go back to the page I want to reply, and this time it works.