I'm having the same problem. I'm not logged in Facebook and the fb sigin popup won't show. Instead I get the Unable to load the registration form. You may have previously blocked this app on Facebook. Go to your Facebook privacy settings to unblock this app. (Error: Missing 'redirect_uri' query parameter.)
I've gone in to the Facebook developers, reset the App Secret, copied and pasted it in the Ultimate Facebook plugin> Facebook API > Secret Key. I've even tried it by unchecking "Preserve individual blog API settings" under the Network Options of Ultimate Facebook settings.
I cleaned my cashed and cookies and nothing.