Starting Mailman on CPanel Servers

# /usr/local/cpanel/3rdparty/mailman/bin/mailmanctl start
Starting Mailman's master qrunner.
# pgrep mailman
30193
# ps auxfw |grep mailman

Make sure that Mailman is enabled in CPanel Tweak Settings Wink.

How to fix this error in Mailing list
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Bug in Mailman version 2.1.9.cp2

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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If you are the server administrator, proceed with the following steps:

1. Go to /usr/local/cpanel/3rdparty/mailman
# cd /usr/local/cpanel/3rdparty/mailman

2.# ls -al

3. # chmod -R 2775 ./*

Check if this has fixed the issue.
If not run the fixmailman script.

4. # /scripts/fixmailman

This should be resolve your issue.