We never received the log showing a failed transfer.
However, as I wrote in the comment, this might be a common issue where a router or firewall drops the FTP control connection that stays inactive during the transfer. To address that, try enabling the keep--alive option:
ftp.Settings.KeepAliveDuringTransfer = true;
ftp.Settings.KeepAliveDuringTransferInterval = 600; // send NOOP every 600 seconds
Please give this a try and let us know whether it helps. If it doesn't, please post the log.