Are you getting the following message in your backup report?
Code:
Could not read reply from control connection: Connection timed outncftpput /home/tmp/admin/user.reseller.name.tar.gz: could not send file to remote host.
I think this is an issue with the remote ftp server you are trying to backup to. I've had the same issue in the past on some ftp servers when backing up large files (>25GB), but I never really looked into it since the problem went away when I switched backup server.