While setting up the Mailman suite in a test environment, the current [mailman-hyperkitty.cfg](https://gitlab.com/mailman/mailman-hyperkitty/blob/master/mailman-hyperkitty.cfg) misled me. Unlike specified in the example, `base_url` actually defines how the archiver can be accessed externally, as [explained](https://lists.mailman3.org/archives/list/mailman-users@mailman3.org/message/A2F65L4IGJZER4BJ3IJUXLSY4GKVD5DT/) by @msapiro. I have created merge request https://gitlab.com/mailman/mailman-hyperkitty/merge_requests/6 with a possible fix.