As with all things Drupal, something always has to go wrong. This happens when visitors aren’t logged in to virtualmin.com and they go to the new forums and try to login from there. I don’t know how to fix it yet.
It’s not redirecting back to the old forum, it’s just failing to redirect back to the new forum after login. It seems to be a bug in the Drupal Discourse plugin, but it’s also plausible I’ve made a mistake in the configuration of it or in Discourse (but, there’s only two options in the Drupal plugin, I don’t know how I could get it wrong).
I’m still trying to figure it out. But, for now, just clicking “Forums” again after logging in at virtualmin.com will take you to the right place and you’ll be logged in.