DM: problem with http call

I am still having a mixed content problem on Pro Sites. The problematic URL is: http://usingess****.com/wp-admin/admin-ajax.php

When you fill the form and click on Reserve your site, it stucks on Processing and gives a mixed content on the browser console.

This happens with the new version from Domain Mapping (4.4.3.1), disabling the plugin fix the issue!