mailing list archive download not working

Steps to reproduce (using torbrowser, but probably also any other browser):

• login to any private ML on lists.tpo

• click on the Archives URL e.g. https://lists.torproject.org/cgi-bin/mailman/private/bad-relays/

• in the last column ("Downloadable version") choose any entry -> right click -> "Save as..."

• try to open the file (it is an html file instead of a .gz) and it does not contain any emails (just the html authentication form)