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.
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