It’s hard to guess since Virtualmin isn’t supported here. You should ask the people who are using Virtualmin if they have solved that problem. Glancing at the topic you linked, though, it appears that everyone else has had trouble with https.
Maybe this is the issue you’re trying to solve? Once your browser has been told to insist on https, it’s hard to change its mind. You might be able to clear your, uh, stuff, to get it to forget the 301 redirect. You can try an incognito window or another browser.
Discourse won’t work without https, though, so you likely still won’t have a functioning site if you do manage to deactivate https.