I had this problem too, and am currently testing with the add_action() call commented out.
So far, so good..
It’s from the file: sfc-register.php
//add_action('sfc_login_new_fb_user', 'sfc_register_redirect');
function sfc_register_redirect() {
wp_redirect(site_url('wp-login.php?action=register', 'login'));
exit;
}
YMMV