Apparently my nightly mailing list hasn’t allowed new subscribers or unsubscribes for about three weeks (or more) now but was still sending out e-mails on schedule so I assumed it was still working. After some experimentation, it was totally broken. I just didn’t read the release notes properly, so I’ll try to be fixing this as soon as I can.
Update: Fixed the sub/unsub problem, so you can now start using the form on this page again to sign-up (or just go here.
Final Update: Got a new version of my scheduled mailer from the developer, and hopefully things should be fixed again. If anybody has any trouble, let me know.