WordPress HTTPS problem with admin-ajax.php

Hi,
i followed the instructions in this post: https://wordpress.org/support/topic/plugin-wordpress-https-ssl-wordpress-https-breaks-ajax-calls-to-admin-ajaxphp#post-4209119
installed the plugin version 3.4.0b but I still see this problem:

Mixed Content: The page at ‘https://www.mywebsite.com’ was loaded over HTTPS, but requested an insecure XMLHttpRequest endpoint ‘http://www.mywebsite.com/wp-admin/admin-ajax.php’. This request has been blocked; the content must be served over HTTPS.

This happens when I submit a form, and that error prevents the form from being submitted.

Any advice?
Thanks

https://wordpress.org/plugins/wordpress-https/