Found that a recent update of our Plesk 9.5.4 server had broken the scheduled backups although manual backups were still working fine. Its seems that after updating PHP to 5.3 and MySQL to version 5.5 the Plesk Backup Manager was erroring with :

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

IgorG at the Parallels forums, highlighted that it was caused by incompatibility between backupmng and MySQL 5.5. To prevent unsigned int overflow following MySQL query he suggested the below command would help correct this issue:

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

So far so good – it seems that scheduled backups are working again, and no other issues to report!