I tried out the 4.2.1 beta release and found out that it is impossible to send files larger than 65536kB. The error- message told me that it is a timeout, but this cannot be. After uninstall that version and installing the stable 4.1.9, everything worked fine, so I think that this might be a bug.
The transmission was pure FTP.
System: WinXP MCE 2005
Receiver: external ftp-disk on a FritzBox- Router.
Hope that this was helpful.