Following up on my post, support responded some 12 hours after the initial trouble report:
Very sorry about that. We introduced a new mailman service version
recently, and some bugs apparently needed to be still worked out. I got
the 'this is for sure fixed' from our developers, so you shouldn't have
any more issues sending mail to the list.
Let me know if there is anything else I can do for you.
My problem appears to be fixed.