The following error can be received by email when the cron to update Mailman's cache is run:
Could not open archive mbox dir for MAILING_LIST: No such file or directory at /usr/local/cpanel/Cpanel/Mailman/DiskUsage.pm line 150.
When your Mailman mailing list's archive is missing the "mbox" directory for a mailing list, the above error can be generated when the cron for Mailman is run. Replacing or recreating the directory should correct the error.
Create and modify the ownership of the missing folder with the following commands (replacing your mailing list name):
mkdir -p /usr/local/cpanel/3rdparty/mailman/archives/private/MAILING_LIST.mbox
chown -v mailman. /usr/local/cpanel/3rdparty/mailman/archives/private/MAILING_LIST.mbox