.Net FtpWebRequest fails sometimes

Question

I try to list file details using FtpWebRequest but very frequently it fails with a WebException and shows error 530 User not logged in.

How is this possible, that it works some of the time using the same credentials?

Excerpt from code:

        reqFTP = (FtpWebRequest)FtpWebRequest.Create(new Uri(ftpuri));
        reqFTP.UseBinary = true;
        reqFTP.Credentials = new NetworkCredential(userName, password);
        string[] downloadFiles = new string[0];
            reqFTP.Method = WebRequestMethods.Ftp.ListDirectoryDetails;
            WebResponse response = reqFTP.GetResponse();
            StreamReader reader = new StreamReader(response.GetResponseStream());
            downloadFiles = reader.ReadToEnd().Replace("\r\n", "¤").Split('¤');
            reader.Close();
            response.Close();