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Glad to see you figured it out 🙂

How are you serving pages over https? Via a plugin or wp-config setting? Apparently your way is not also changing the URL of admin-ajax :/

Strange. As you can see in our source code, we try to detect the URL schema for the CDN:

https://plugins.trac.ww.wp.xz.cn/browser/wp-slimstat/trunk/wp-slimstat.php#L1413

for some reason is_ssl() is returning false for admin-ajax requests. Can you contact their support and see if there’s anything they can do about it?

That’s what Slimstat already does (is_ssl) 🙂