I'm guessing that based on the timelapse since originally posting this, you've been able to determine the cause of the issues, but we ran into this as well. The problem is with freeFTPd. It has a file size limitation (1GB) and will not complete the backup if the file is larger. We've been looking for another sftp server that will work (with little success to this point), and had opened a case a while back. We were advised to use OpenSSH via Cygwin (for backups to a windows server).
It would appear that there is certainly a file size limitation within FreeFTPd which Cisco are aware of, unfortunately this is not highlighted with the documenation regarding DRS which I can find. The following is taken from the CUCM Managed Service Guide 7.1 regarding SFTP Billing servers below:-