[Mailman] Error message displayed in subscribe process

[Satoshi IIDA]

Hello ML admin,

Thank you for keeping the environment healthy!

I'm a Mailing list operator of Talk-ja.
A user got an error message when the user subscribes there.
(And failed to add him.)

https://lists.openstreetmap.org/listinfo/talk-ja

As I add him using the mailman management page, I got the same error
message.
Do you have any insight on this?

## Error message
    Bug in Mailman version 2.1.29
    We're sorry, we hit a bug!

    Please inform the webmaster for this site of this problem. Printing of
traceback and other system information has been explicitly inhibited, but
the webmaster can find this information in the Mailman error logs or the
web server logs.

## the mail address I tried to add
    open_ffff23 at protonmail.ch

Best regards,

-- 
Satoshi IIDA
mail: nyampire at gmail.com
twitter: @nyampire
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