I search for the data-rsssl=1 in the source code but its not there. The site in question is a one page site, not multiple pages. The icon in the URL address bar shows a black colored lock and a yellow caution icon. How can I get this fixed? The server runs the Let’s Encrypt SSL Cert.
If you the marker is not there, this means the mixed content fixer is blocked by another plugin. I would first try to switch the mixed content fixer hook in settings/ssl, if that doesn’t help, you can try deactivating some of your plugins, to see which one is blocking it.