Agents need to be "wakened" before backups succeed.

[RSTJNII]

If the backup agents are not "awake" the backup cron job will fail with a 403. Before the StartManual request is made the /user/wakeupagents endpoint should be hit for the account. See http://docs.rackspace.com/rcbu/api/v1.0/rcbu-devguide/content/Wake-Up_Agents-d1003.html

This likely warrants a refactor of the /etc/driveclient/run_backup method.