在WordPress多站点用户前端页面中POST“admin-ajax.php”400

Question

I'm trying to use AJAX in the user frontend pages from a plugin in WordPress Multisite but the admin-ajax.php generates Bad Request 400 and always fails.

1. Developing WordPress Plugin - AJAX in admin pages working good
2. WordPress Multisite
3. Test on Site_1

This is a part of my code and the output, I just shorten it, There's a class to instantiate the AJAX actions and another one after it to include the JS file using wp_enqueue_scripts action ..

Anyway the JS file included correctly and console.log function displaying all JS data object as you see in the code below, BUT it always fails and msg argument print Bad Request in statusText .. What I missed??

/* == AJAX Class == */
add_action( 'wp_ajax_wpmu_frontendpage', array( &$this, '_class_function' ) );
add_action( 'wp_ajax_nopriv_wpmu_frontendpage', array( &$this, '_class_function' ) );


/* 1. The JS file included correctly */
wp_register_script( 'wpmu/scripts/frontend/core', 'JS_FILE.js', 
    array( 'jquery' ), '1.0', true );

/* 2. Localize */
wp_localize_script( 'wpmu/scripts/frontend/core', 'wpmu_ajax', array (
    'ajax_url'  => esc_url( admin_url('admin-ajax.php') )
  )
);

/* 3. Enqueue */
wp_enqueue_script( 'wpmu/scripts/frontend/core' );


== JS FILE ==
(function ($) {
'use strict';
$.wpmu = $.wpmu || {};
$.wpmu.ajax_vars = {
    ajax_url: wpmu_ajax.ajax_url,
};
$(document).ready(function () {
    /* 1. DEBUG */
    console.log($.wpmu.ajax_vars);
    /* 2. DEBUG - OUTPUT */
    -- ajax_vars:
    --    ajax_url: "http://localwp.com:90/wpmu/SITE_1/wp-admin/admin-ajax.php"

    /* ========= Main Code ======== */
    $(document.body).on('click', '#my-wrapper .submit', function (e) {
        e.preventDefault();

        var server_data = {
            action: 'wpmu_frontendpage',
        };

        $.ajax({
            method: 'POST',
            async: true,
            url: $.wpmu.ajax_vars.ajax_url,
            data: server_data,

        }).done(function (response) {
            alert('success');
        }).fail(function (msg) {
            console.log( msg );
        });

        return false;

    });
});

})(jQuery);