Disregard my previous comment; my problem turned out to be an issue with my external firewall.
Having the same issue. Connection timeouts to webmin over HTTPS to port 10000. Using wget locally on the server gives me the login page, but unable to reach it remotely. Most recent changes were updating webmin this past weekend (I update monthly), and installing a new LetsEncrypt certificate on one of my vhosts. I wonder if Brian is on to something. /var/webmin/miniserv.log doesn't show the connection attempt and /var/webmin/miniserv.error isn't logging anything either.