Recently, somebody has reported that my web site is not secure, however the certificate has been installed a long ago and the setup has been working nicely in the past.
I tested that only Brave Desktop is throwing the warning (see images below), but it works fine with my other browsers (Opera, Firefox, Chrome). I wasn't able to verify the browser of the person who has reported the warning.
Brave has stricter settings regarding privacy protection than some other browsers do, as a result certain things will fail in Brave that will pass in other browsers.
Currently your homepage seems fine though, if you click other pages such as About or Story the developer tools console is showing various ad trackers being blocked but these shouldn't result in "Not Secure".
You should review the dev tools console to see if it's complaining of something specific.