If you are getting following error when trying to modify mailinglist then execute following command on shell
Error
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Bug in Mailman version 2.1.11.cp3
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.
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Command
chmod 02775 -R /usr/local/cpanel/3rdparty/mailman/