C# WebClient DownloadString() throwing exceptions

Question

I'm trying to download files using the C# WebClient method DownloadString(), but it throws an exception every time.

Every attempt is causing this exception: "The underlying connection was closed: An unexpected error occurred on a send." The inner exception is: "Authentication failed because the remote party has closed the transport stream."

But these URLs work perfectly if I enter them in a browser. Any ideas why DownloadString() is failing?

The code:

class Program
{
    // URI pattern:          https://www.nndc.bnl.gov/nudat2/decaysearchdirect.jsp?nuc=235U&unc=nds
    static string baseUri = "https://www.nndc.bnl.gov/nudat2/decaysearchdirect.jsp?nuc=";
    static string destFolder = @".\Downloaded Files\";
    // Create a new WebClient instance.
    static WebClient myWebClient = new WebClient();
    static XElement isotopes = XElement.Load(@"..\..\..\..\MakeDecayChainLinks\MakeDecayChainLinks\bin\Debug\isotopes2.xml");
    static StreamWriter sr = new StreamWriter("Log.rtf");

    static void Main(string[] args)
    {
        foreach (XElement e in isotopes.Elements().Reverse())
        {
            string isotopeName = e.Attribute("AtomicWeight").Value + e.Attribute("Symbol").Value.ToUpper();
            string uri = baseUri + isotopeName + "&unc=nds";
            string destFile = destFolder + isotopeName + ".txt";
 
            try
            {
                string content = myWebClient.DownloadString(uri);    // <=== THIS IS WHERE IT FAILS.
                using (StreamWriter sw = new StreamWriter(destFile))
                    sw.Write(content);

                sr.WriteLine(isotopeName + " downloaded\nURI: " + uri);
            }
            catch (Exception ex)
            {
                sr.WriteLine(isotopeName + " NOT FOUND: " + ex.Message + "\nURI: " + uri);
            };
            
            Console.WriteLine(uri);
        }

        sr.Close();
        Console.ReadKey();
    }
}