Error code 66

I am using Comodo Backup Free v.2.2.127000.12 on Windows 7 Home Premium machine. I successfully set up & manually ran a scheduled backup to an external hard drive over a workgroup network. However the subsequent automatic scheduled backups failed with error code 66.
Can someone please tell me what that means, or if there is a list of all error codes (not just the most common).
Thanks

This might be an error in closing network connection.

Thanks