I unfortunately had to manually fix this problem right away, so it's not sitting there for us to look at. But here's an issue I ran into last night.
I moved a sub-server to a new private IP address, then enabled the "SSL website enabled" feature.
However, while it did act as if it did that, and I saw no errors, it did not actually add the SSL Virtual Host on port 443.
I manually added a VirtualHost section for it -- but I later learned that I also had to go into /etc/webmin/virtual-server/USERNAME.acl, and enable:
edit_ssl=1
feature_ssl=1
Both were previously unset.
Manually setting the above options, as well as adding the Virtual Host entry, resolved the issue.
SSL has worked just fine on other sites here -- I notice the ones that worked were all top-level Virtual Servers though, where this is a Sub-Server.
Also, this Sub-Server has a different IP than it's parent server has.