FTP in a nutshell . . .

FTP client communicates to FTP host via TCP/IP to transfer files.

Transfer errors are "typically " caused by TCP/IP communications errors (but not always.)

Session errors are trickier to diagnose.

i.e. FTP server settings "could" reset the session after a specified time.

Best bet is to enable logging on the server (enabled from the properties of the ftp server site)

and recreate the error / conditions causing the error. (then review the logs.)

Offhand . . . Why FTP?

It's not a bad solution, but the overhead of running a FTP server seems wasteful.

Maybe consider Robocopy as a solution?

http://ss64.com/nt/robocopy.html

e.g. Map a drive (assuming windows environment) and use robocopy to transfer the backup file(s).

(Far easier to setup & diagnose than a full FTP client/server solution.)

M