Link: http://www.myoracleportal.net/blog1.php/2009/02/01/plsqlftp

Author Comments : Important issue identified regarding the structure of your remote FTP servers banners. Although with a true FTP client tool it may not be required, this FTP process follows best practice and expects that the server codes precede each line being returned by the remote FTP Server as it authenticates to the remote server. Failure to include the required prefix could cause you to be unable to access the remote server or failure to authenticate. See for more info => TCP Banners and then contact the respective server administrator.

Additional information on server codes can also be found here.

It might be possible to enhance the PLSQL solution to work past the issue, however I would not encourage that and instead stay with best practice.