You might have received the following error while accessing your mailing list after recent cPanel update:
Bug in Mailman version 2.1.12.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.
In most cases, the above error was occurred due incorrect permissions set for mailman directory. You can run following command to change the permissions via SSH:
chmod 02775 -R /usr/local/cpanel/3rdparty/mailman/
Hopefully this will resolve your issue. If the problem persists, you can open a ticket with cPanel support. They should help you to resolve this.