[MxxCon]

during regular ftp session when you transfer a file, ftp client will send a PORT command that should contain your internet IP. some ftp clients don't know the difference between internet ip and local ip, so they will mistakenly send PORT command with an ip from your lan(192.168.0.x or whatever other range you use). your router thinks its helping you by modifying this PORT command with any ip address to your internet ip, allowing downloads or uploads to work.
during FXP transfers you know what PORT command should contain, however your router doesn't, so it still blindly modifying that command.