My experience has been once the Nightly Cleanup begins, it doesn’t stop until it has been completed. We have over 100 machines backing up on our server and to accommodate the time I have changed my schedule to backup Monday - Friday 0-24, and then on Friday at midnight to Sunday midnight no backups are allowed and the cleanup occurs. This seems to have helped my situation. Our server is configured for internet backups only, we have no machines on a local network being backed up.