After some digging I found the problem: 301 redirects don’t pass POST parameters. And when I configured the mailing list I set the domain to nkuttler.de. So mailman sets the form action to http://nkuttler.de/foo/bar, this gets redirected to the correct domain, but the POST data is gone. How do you change the web host? I couldn’t find anything in the admin interface, it looks like you have to do it on the command line.

cd /var/lib/mailman/bin
./withlist -l -r fix_url list -u www.nkuttler.de/

This is the precise command for Debian, on other distros you’ll have to find the correct location.

Update: I’m not quite sure how I missed it, but the host_name parameter should allow you to change this setting as well. For other questions about the default URL you can also read the Where can I change a list or the default URL used for the web interface? FAQ entry.