I've just encountered some problems with SSL (HTTPS) and IPv6. I installed Let's Encrypt certificates on most of the websites using the "Manage SSL Certificates" in Virtualmin (thanks for that! :) ) and configured a redirect for all HTTP traffic to HTTPS. I noticed that in some cases, the websites would cause an error (ERR_SSL_PROTOCOL_ERROR) in my browser. After some troubleshooting, I found that there is no HTTPS respons on IPv6. This is caused because of the configured listening address for port 443, which is the IPv4 address for the server. I couldn't find an option to change this in Virtualmin, so I did it using a text editor.
The /etc/apache2/sites-available/*.conf files contained a line, which I changed to . This fixed the problem.
Virtualmin should make it *:443 by default, so IPv6 is also supported.