>>993
ググってみましたら、同じような事象があったので、このサイトの通りに書き換えたDownloadがこちらです。
https://qiita.com/muro/items/8b9f5886c56f7b6afd3c

private void Download(int date, string url, string filename)
{
var uri = new Uri(url);
try
{
using (var wc = new System.Net.WebClient())
{
var namePassword = string.Format("{0}:{1}", SITE_ID, SITE_PASSWORD);
var chars = System.Text.Encoding.ASCII.GetBytes(namePassword);
var base64 = Convert.ToBase64String(chars);
wc.Headers[HttpRequestHeader.Authorization] = "Basic " + base64;
//webClient.Credentials = new NetworkCredential(SITE_ID, SITE_PASSWORD);
using (var reader = new StreamReader(wc.OpenRead(url), System.Text.Encoding.GetEncoding("shift_jis")))
{
var line = reader.ReadToEnd();
if (line.Trim() != string.Empty)
{
System.IO.File.WriteAllText(filename, line, System.Text.Encoding.GetEncoding("shift_jis"));
}
}
}
}catch(Exception ex)
{
Console.WriteLine(ex.Message);
}
}