I have a proxy server in front of Apache. Virtualmin knows about this configuration because I changed the listen ports inside Virtualmin interface. Then I changed ports in /etc/apache2/ports.conf too. Everything worked perfect before updating Webmin to 1.770. After updating Webmin apache2 service did not start. I have experience with this issue. It was easy modifying again ports and NameVirtualHost in apache2.conf then restarting the service.
Webmin update changes ports in Apache