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.)
Free Expert Advice . . . http://xkcd.com/627/