When I setup my backup for FTP I'd watch the log and it would backup my site, then authenticate properly to my ftp server. It would then fail on the upload. I tried passive on and off (which made no difference by the way it used passive regardless of the switch on or off).
I noticed it allowed dropbox backup, but it wasn't making that available because curl wasn't installed on my server. So I installed curl and setup drop box. That worked fine.
It got me to thinking that maybe ftp failed because curl wasn't installed. After all who wants to pay dropbox for storage they already have somewhere else... So I re-ran the ftp backup I had setup and guess what... IT WORKED :)
So the solution is MAKE SURE YOU HAVE CURL INSTALLED on your server if you are having FTP upload fails.