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.
ERR_SSL_PROTOCOL_ERROR
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