This is a way belated response, but since this is the only result which turns up when you search this error, I'm going to add it anyway. In short, when you get this error:
Aug 14 13:43:26 2019 (28962) Uncaught runner exception: Syntax error: Expected authserv-id at: ; dkim=pass header.d=domain.tld; arc=none... ... you've most likely missed to add the following to your mailman.conf: [ARC] authserv_id: yourdomain.tld
Mailman, unfortunately, doesn't verify that the [ARC] configurations are complete, and the documentation for enabling ARC-authentication is, as you noted, deceptively simple. The [ARC] chapter in the documentation for all configuration options (https://docs.mailman3.org/projects/mailman/en/latest/src/mailman/handlers/do...) is much better, albeit a bit terse if you just want a minimal example.
The Arch wiki (https://wiki.archlinux.org/title/mailman#ARC) has a near-minimal example, but no explanations.