It looks like the server does not support username and password login only. In addition you have to use your private key for the authentication as shown below:
Sftp sftp = new Sftp();
sftp.Connect("servername", port);
// try using either
sftp.Login("user", new SshPrivateKey("pathToPrivateKey", "keypassword"));
// or method
sftp.Login("user","password", new SshPrivateKey("pathToPrivateKey", "keypassword"));
to be able login to this SFTP server.