I'm trying again after my previous mailman failures. I've learnt that mailman just does not work with virtualhosts, because it hardcodes one user and each virtualhost runs scripts as a different user, and because virtualhosts (because of suexec) refuse to run scripts from common places like /usr/lib/mailman/cgi-bin/.
So I've created a virtualhost (which requires a subdomain, but lists.gnome-ev.de is OK to have anyway) just for mailman and installed mailman from source in the virtualhosts's allowed cgi-bin path.
However, it still complains that it's not running as the www group, even though I configured it from source like so:
./configure --prefix=/srv/www/htdocs/web10/software/mailman --with-cgi-gid=ftponly --with-mail-gid=ftponly --with-groupname=ftponly --with-username=web10