The remote server returned an error: (401) Unauthorized

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
//using System.Text.RegularExpressions;
using System.IO;
using System.Net;
//using System.Net.Configuration;
//using System.Web.Services;
//using System.Web;
//using System.Collections;
//using System.Web.Services.Protocols;
//using System.Web.Script.Services;
//using System.Xml;

namespace ConsoleApplication27
{
    class Program
    {
        static void Main(string[] args)       
        {

// Create the web request  
            try
            {
                HttpWebRequest request = WebRequest.Create("https://ERS123.optimahcs.com/cpapi/v2/clinical/patients/12377") as HttpWebRequest;
                request.Method = "GET";
                request.Credentials = new NetworkCredential("ERS123clinicalapi", "###########");
// the next line generates exception [System.Net.WebException] = {"The remote server returned an error: (401) Unauthorized."}
                using (HttpWebResponse response = request.GetResponse() as HttpWebResponse) 
                {
                    // Get the response stream  
                    StreamReader reader = new StreamReader(response.GetResponseStream());

                    // Console application output  
                    Console.WriteLine(reader.ReadToEnd());
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
        }
    }
}