SQL Backup failed with exit code: 669

Anybody know what a 669 is?
the "Error code reference - SQL Backup" page here skips from 668 to 680.

http://www.red-gate.com/supportcenter/content/SQL_Backup/help/7.2/SBU_Errorcodereference

The version i'm running is 7.4.0.23
Thanks!
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Date 11/11/2013 8:43:36 AM
Log Job History (SQL Backup differential backup: model and 6 others (created 10/24/2013 3:44:44 PM))

Step ID 1
Server BB1DB
Job Name SQL Backup differential backup: model and 6 others (created 10/24/2013 3:44:44 PM)
Step Name execute master..sqlbackup
Duration 00:00:00
Sql Severity 16
Sql Message ID 50000
Operator Emailed
Operator Net sent
Operator Paged
Retries Attempted 0

Message
Executed as user: CWI\sql-sa-bb1db. SQL Backup failed with exit code: 669 SQL error code: 0 [SQLSTATE 42000] (Error 50000). The step failed.