Hello everyone,
Jarkko complained me that the Nerds' business lunch list was not delivering mail some time ago. I had upgraded the server to Debian Bullseye and it seems the Mailman 2 package is gone, and only Mailman 3 is supported nowadays.
So I've finally installed Mailman 3 and migrated the old list.
If you get this e-mail, the list should be operational again.