Mailman lists are missing from cPanel

Symptoms

cPanel is accessed to work with existing mailing lists but they do not display in the cPanel user interface.

Cause 

There are a few reasons why Mailman lists may not be showing in cPanel. The main issue is due to one of the lists being corrupted. This is able to be seen by executing the following:

# /usr/local/cpanel/3rdparty/mailman/bin/list_lists

If you receive an error from the above command, one of the mailling lists is corrupted.

Resolution

To recover this file, you will want to restore these from a recent cPanel backup. 

For more on how to do this, please see the following article:
How to manually extract individual files from a cPanel backup with the tar utility

In the event that you do not have a backup to restore from, you will want to move the list data aside and re-create the mailing list. 

1. Log in as the root user via SSH, or using the Terminal in WHM.

2. Move the old list aside:

   # mv -v  /usr/local/cpanel/3rdparty/mailman/lists/listname_domain.tld{,.$(date +%s)}

   Note: Replace listname_domain.tld with the lists name you want to regenerate.

3. Re-create the mailing list within cPanel.

4. Verify that no lists report any errors with the following command:

   # /usr/local/cpanel/3rdparty/mailman/bin/list_lists