Mixed Content

Hello,

I am getting this message in my console using Slimstat on a secured website:
Mixed Content: The page at ‘https://……..nl/’ was loaded over HTTPS, but requested an insecure XMLHttpRequest endpoint ‘http://www………nl/wp-admin/admin-ajax.php’. This request has been blocked; the content must be served over HTTPS.

According the developers of Slimstat there is nothing wrong with their plugin:
https://wordpress.org/support/topic/mixed-content-3?replies=8#post-6849414

How can I solve this problem?

Thanks in advance!

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