Backup not executed as per schedule in DP 9.09

Backup not executed as per schedule in DP 9.09. 

Saw below error in Data protector Event logs. Restarted the skipped backup and it completed without any issue.

[138:726] CsaStartSession failed for datalist "Backup_spec"

Description:
Backup could not be started because CsaStartSession function failed.
Actions:
* Restart the backup.
* Try running datalist with omnib -session <datalist_name>