Ajax in mu-plugins

I am converting a rating and aggregator plugin I wrote a couple of years ago (that works correctly) to a must-use plugin for a multisite network.

Problem is, although the ajax routine works just fine, when it is supposed to register the results it is not calling the callback function specified in the wp_ajax actions.

In the constructor of my plugin’s code I have:

add_action( 'wp_ajax_ipubstarajax', array( $this,'star_ratings' ) );
add_action( 'wp_ajax_nopriv_ipubstarajax', array( $this,'star_ratings' ) );

Then later in the code scripts and css are enqued and the js is localized:

public function register_plugin_scripts() {

    wp_register_script( 'my_eg_starscript', plugins_url( 'fcn-stars/js/eg-star-rating.js', __FILE__ ), array('jquery'));
    wp_register_script( 'eg-ajax-script', plugins_url( 'fcn-stars/js/eg-star-user.js', __FILE__ ), array('jquery'));
	 wp_enqueue_script( 'jquery' );
	 wp_enqueue_script( 'my_eg_starscript' );
	 wp_enqueue_script( 'eg-ajax-script' );
    wp_localize_script( 'eg-ajax-script', 'myAjax', array( 'ajaxurl' => admin_url( 'admin-ajax.php' )));        

	} // end register_plugin_scripts

In the eg-star-user.js file is the function called by the plugin:

function sendRatingsToServer(){	

   var str = $('form').serialize();
         	$.ajax({
	        type: "POST",
	        url: "/wp-admin/admin-ajax.php",
	        data: 'action=ipubstarajax&'+str,
	        success: function(response) {
///////////////////////  ETC

Am I not calling the URL and ACTION properly for a plugin in the mu-plugins folder??

Any help is appreciated.

Thanks,

Ed