On an otherwise working Virtualmin port 80 HTTP website you may see the following error:
This site can’t provide a secure connection
example.com sent an invalid response.
It may well be that SSL HTTPS was never switched on for this website. You can switch it on using the command line like so:
sudo virtualmin enable-feature --domain example.com --ssl
If you’re a command line junky like me, you’ll also find this line rad:
virtualmin generate-letsencrypt-cert --domain example.com