April 22, 2025
12:30 p.m.
This reply will probably not help you, but... when I encountered a similar error, restarting the server-side Mailman services solved it, and it was a one time issue that didn't recur.
Albeit if it happens again after a restart, or even better, you find a reliable way to reproduce it, I suggest to wait for the response of the pros/devs, as I'm also a new user of Mailman 3.