I have an https domain hosted by Fasthosts which points to a Ubuntu linode server with nginx and letsencrypt on it. This points to an application on an http application on another linode server so that users of the application see everything as SSL.
This works fine pretty much all of the time but sometimes I get a user saying it has been flagged as insecure by a browser (with the https in the url bar in red and crossed out). This is often a larger company who likely has more robust security in place than a general user.
I had all this set up for me a couple of years ago by someone who can no longer be contacted and am hoping to find a solution or someone to help me!
I have tried asking people more technical than me as well as Linode.