Error 501 when connecting to FTP

I have a C # (Windows Forms) application that accesses the company's FTP to download some files and / or upload.

Only, on two specific clients, the application can not access FTP. It gives the following error:

  

The remote server returned an error: (501) Syntax error in parameters or arguments.

I have searched extensively on this error, but there is nothing significant.

What can it be?

My code

string enderecoUltimaVersao = "ftp://meusite.com.br/releases/ultima";

FtpWebRequest request = FtpWebRequest.Create(enderecoUltimaVersao) as FtpWebRequest;
request.Method = WebRequestMethods.Ftp.ListDirectory;
request.Credentials = new NetworkCredential(user, senha);
request.UsePassive = true;
request.UseBinary = true;
request.KeepAlive = false;

FtpWebResponse response = request.GetResponse() as FtpWebResponse;
Stream responseStream = response.GetResponseStream();
StreamReader reader = new StreamReader(responseStream);

while (!reader.EndOfStream)
{
    string file = reader.ReadLine();
    files.Add(file);
}