Ajax request to addthis_check_user_exists is returning 0 (zero)

To reproduce this error just visit http://www.boicotasp.com.br/login and login with Facebook or Google. After that, a Lightbox will appear to complete your registration data, but when you hit Continue, the ajax request is sent and it just returns 0 (zero). It’s not even executing the function, I guess. I think WordPress is not finding the hook, although it is including the file correctly (already tested that).

Thanks in advance.

http://wordpress.org/extend/plugins/addthis-social-sign-in/