My mailman redirects for customers were failing because, of course, they point to the customers’ sites which do not have SSL certificates. I fixed this by manually changing the redirects in httpd.conf to point to a general purpose secure site I run for this purpose.
Where can I go to edit the default values for things like this in a non-volatile way? (read: won’t break when I upgrade virtualmin later) Is this part of the template config somewhere?
You can tweak the settings used for Mailman by going into System Settings -> Features and Plugins, look for where it says “Mailman”, and click the “Configure” link next to that.
On that screen, you can specify what URL to use for the Mailman links that are added into the Apache config.