I have received following information from developers:
Issue occurs on Plesk with postfix.
Root cause:
'/usr/local/psa/admin/bin/mailmng --features' fails to run 'postconf' if it isn't in PATH as for scheduled backup.
Workarounds:
1. Back up domains w/o suspending - error about sitebuilder still occurs.
2. Suspend/unsuspend any domain to set 'mailmng:features' in 'BackendCache' table.
3. Set PATH to 'postconf' to /etc/cron.d/plesk-backup-manager - typically it's a '/usr/sbin/'.