Comment 1 for bug 1346468

We need to see the error information from Mailman's 'error' log to diagnose this. The log is at /usr/local/cpanel/3rdparty/mailman/logs/error . If you are on a hosted server and don't have access, you will have to report this to the admins of the hosted server as only they will have the access necessary to diagnose and fix this.