I would recommend you use the hostname of one of your Virtualmin domains to connect to Usermin (and Webmin). That gets you an https connection with a valid TLS cert.
You’ll probably want to look at the Web Developer tools in your browser to see what is happening. Maybe a JavaScript or network error.
If nothing is happening in the browser, you might check the Usermin miniserv.error log in /var/usermin. Though I kinda doubt this will be a server-side issue.
This is a Virtualmin system right? It would normally setup Usermin for mail in Maildir or via IMAP (I don’t remember which), not a mail spool like that.
yes, but the key question is HOW? and was it a clean OS?
there are some pretty useless instructions out there in the wild - not to mention some OS provided by 3rd parties that are not clean