Resolution
SYMPTOMS:The following error occurred in Plesk CP when you access Domain->Mail->Mailing List->LIST_NAME:
Unable to init MailListMembersList object: Unable to get list members: Unable to get members list: listmng failed: listmng: Error: Unable to list members of mail list LIST_NAME.
RESOLUTION:
Make sure that the list directory exists and the list database is not corrupted:
# ls -al $MAILMAN_VAR_D/lists/LIST_NAME
# $MAILMAN_ROOT_D/bin/check_db --all
$MAILMAN_VAR_D and $MAILMAN_ROOT_D variables can be found in /etc/psa/psa.conf.