My site is hosted with WP Engine. When I view the site, I get a "mixed content" SSL message that states that "wp-content/plugins/s2member/images/ajax-loader.gif" isn't being served over HTTPS.
I got WP Engine tech support on it. They were able to determine that the instructions for loading this image are coming from relative paths within files in S2Member's plugin folder, which I was also able to find myself.
However, I don't know how to resolve this issue.
And I've found that others have had this issue too (Googling "s2member mixed content" brings up a number of results regarding the same issue)
WP Engine support suggested that I reach out to S2 Member.
When I run the page through whynopadlock.com it also shows that it's only that one call on the home page that's made insecurely, you can see this at https://www.whynopadlock.com/results/cf688e2b-17d7-4f42-bc31-099009df8d55
preloadAjaxLoader = new Image (), preloadAjaxLoader.src = '<?php echo $vars["i"]; ?>/ajax-loader.gif';
But I'm not a developer...and don't think it's a good idea to go fiddling in the core files.
What I've tried...
Updated to the latest version of S2Member (170722)
Installed the SSL Insecure Content Fixer
change to wp_config.php: https://codex.wordpress.org/Function_Reference/is_ssl