I can not re-enable or change sending of mailing list mail to
Internal Server Error
I've managed to reproduce the error, it apparently happens when
someone has a lot of subscriptions (say, more than 15) because the
admin UI uses a not-scalable way of getting that information.
I'll look into it and see if we have the necessary endpoints in
Mailman's REST API to do better, or if I can change the page structure
to avoid this very long request.
Thanks for reporting.