WordPress installation in a subdirectory causes the Translation Tools tab to not load properly

Overview of the issue

If your WordPress installation is in a subdirectory, you may find that the Translation Tools tab is empty when you go to WPML → Translation Management.

When you open the browser console, you see an error similar to:

Mixed Content: The page at ‘/wp-admin/admin.php?page=wpml-translation-management%2Fmenu%2Fmain.php&sm=ate-ams’ was loaded over HTTPS, but requested an insecure script ?wpml-app=ate-widget&ver=2.11.0’. This request has been blocked; the content must be served over HTTPS.

Workaround

This issue comes from a server configuration, and we recommend that you contact them. When your server requests a URL without a trailing slash (e.g. https://example.com/sub-directory), it redirects to a URL with a trailing slash but without an HTTPS (e.g http://example.com/sub-directory/).

In the meantime, you can use the following workaround:

1. Make a full backup of your site before proceeding.
2. Open your functions.php theme file.
3. Add the following snippet:

   add_filter('site_url', function ( $site_url ){
   	return rtrim( $site_url, '/' ) . '/';
   });