This messages indicates that an FTP server unexpectedly closed a connection.
There were reports1 that the disconnect is sometimes caused by a Windows firewall on a local machine or on a remote server (if the server runs on Windows), if a stateful FTP filtering is enabled on the firewall.
To disable the stateful FTP filtering, in an Administrator command prompt, execute following command:
netsh advfirewall set global StatefulFTP disable
For details refer to How to Configure Windows Firewall for a Passive Mode FTP Server.