Make sure the AJAX URL for EDD uses the protocol for the current page, either http or https, as it doesn't work otherwise.

EDD_AJAX_non_ssl_filter.php

<?php

/**
 * Make sure the AJAX URL for EDD uses the protocol for the current page, either http or https, as it doesn't work otherwise.
 *
 * @param string $ajaxurl The current ajaxurl
 *
 * @return string
 */
function yst_filter_edd_ajax_url( $ajaxurl ) {
    if ( is_ssl() ) {
		return $ajaxurl;
	} else {
		return str_replace( 'https://', 'http://', $ajaxurl );
	}
}

add_filter( 'edd_ajax_url', 'yst_filter_edd_ajax_url' );