This fixes 403s for ajax login plugins.

fix-login-403.php

<?php
/**
 * Plugin Name: Fix Ajax Login on WP Engine.
 * Description: Removes the wpe_login_failed_403 action.
 * Version: 0.1
 * Author: Jason Stallings
 */
 
 // Prevent direct access to this file
if ( ! defined( 'ABSPATH' ) ) {
	die( "You can't do anything by accessing this file directly." );
}

//Remove our action that 403s on failed login.
add_action( 'plugins_loaded', 'jstall_remove_login_failed');

function jstall_remove_login_failed()
{
        remove_action( "wp_login_failed", "wpe_login_failed_403");

}