From what I see, wpdebat.dk works on my side. You have a valid SSL certificate for root and www. You still have mixed content; as the guide says, you need force https to fix that.
Is it wanted to use www.wpdebat.dk instead of wpdebat.dk in DISCOURSE_HOSTNAME?
I don’t know if it can cause an issue, but I would use wpdebat.dk and tell Virtualmin to redirect www → non-www (or the inverse if you prefer to keep www).
Since you’re using www in Discourse config, that would cause trouble.
I would still encourage you to choose between www or non-www, adjust app.yml, and make a redirection in Virtuamin. That should give you SEO benefits (consistency in the URL, preventing content duplication, analytics under a single domain, etc).