This is tricky for a few reasons and will likely require you to understand an apache conf file and http connection details better.
Most requests use port 80 for http access. When you type http://example.com in your browser, your machine makes a connection on port 80 to example.com and will probably find a web server running on that machine, listening for connections on port 80.
If you type https instead of just http, (https://example.com) then your machine makes a connection on port 443 instead. Most default apache configurations are set up such that a connection on port 443 will result in your server sending a certificate to the requestor. Due to the nature of SSL connections and the HTTPS protocol, you can only have one SSL certificate on port 443 on your server.
While http assumes port 80 and https assumes port 443, the http protocol allows you to also specify a different port in the url. In this example, port 10000 is specified: http://example.com:10000
I'm not certain, but it sounds like webmin might be hosting on some non-standard port (i.e., not port 80 or 443) and the apache configuration is such that it serves up some certificate. You should be able to use a different cert for the standard port 443.
Depending on how you installed apache on your system, you may or may not have a certificate already set up. Have you tried simply requesting your files via https? what happens?
