The remote certificate is invalid according to the validation procedure

// C# version

using (Ftp client = new Ftp())
{
    // Use custom certificate validation:
    client.ServerCertificateValidate +=
        new ServerCertificateValidateEventHandler(Validate);

    // Minimalistic version to accept any certificate:
    // 
    //client.ServerCertificateValidate +=
    //    (sender, e) => { e.IsValid = true; };
    // 

    client.ConnectSSL("ftp.example.org");
    client.Login("username", "password");

    foreach (FtpItem item in client.GetList())
    {
        if (item.IsFolder == true)
            Console.WriteLine("[{0}]", item.Name);
        else
            Console.WriteLine"{0}", item.Name);
    }
    client.Close();
}

private static void Validate(
    object sender,
    ServerCertificateValidateEventArgs e)
{
    const SslPolicyErrors ignoredErrors =
        // self-signed
        SslPolicyErrors.RemoteCertificateChainErrors
        // name mismatch
        |  SslPolicyErrors.RemoteCertificateNameMismatch;  

    if ((e.SslPolicyErrors & ~ignoredErrors) 
        == SslPolicyErrors.None)
    {
        e.IsValid = true;
        return;
    }
    e.IsValid = false;
}

' VB.NET version

Using client As New Ftp()
    ' Use custom certificate validation:
    AddHandler client.ServerCertificateValidate, AddressOf Validate

    client.ConnectSSL("ftp.example.org")
    client.Login("username", "password")

    For Each item As FtpItem In client.GetList()
        If item.IsFolder = True Then
            Console.WriteLine("[{0}]";, item.Name)
        Else
            Console.WriteLine("{0}", item.Name)
        End If
    Next
    client.Close()
End Using

Private Sub Validate( _
    ByVal sender As Object, _
    ByVal e As ServerCertificateValidateEventArgs)

    Const ignoredErrors As SslPolicyErrors = _
        ' self-signed
        SslPolicyErrors.RemoteCertificateChainErrors _    
        ' name mismatch
        Or SslPolicyErrors.RemoteCertificateNameMismatch        

    If (e.SslPolicyErrors And Not ignoredErrors) = SslPolicyErrors.None Then
        e.IsValid = True
        Return
    End If
    e.IsValid = False
End Sub