We some time might receive following error while doing a backup process via Plesk:

backupmng: Unable to fetch backup tasks from database: BIGINT UNSIGNED value is out of range in '(-(weekday(`psa`.`BackupsScheduled`.`last`)) + `psa`.`BackupsScheduled`.`backup_day`)'

To solve the issue login to the server via SSH and perform following steps

mysql -u admin -p`cat /etc/psa/.psa.shadow`
use psa
ALTER TABLE BackupsScheduled MODIFY COLUMN backup_day INTEGER;

Once done the Scheduled backup should work fine.