WebClient/ HTTPWeb Some files are not downloaded completely

Good day everyone,

We are currently developing an update service that will automatically update our programs. For this, the latest versions of these programs are provided via a link, which can be downloaded after a login. The service is supposed to download these files now... this works partly also, but somehow certain files (always the same ones) are downloaded with only one kilobyte. Here it doesn't matter if it's a .zip or an .exe file, as it downloads one file of each correctly. Also, it does not seem to matter the size, as sometimes larger files are downloaded, but smaller ones are not. If the provided link is called manually, the files that could not be downloaded automatically can be downloaded normally. For the automatic download, we have already tried it with the WebClient:

client.Credentials = new NetworkCredential(username, password);
client.DownloadFile(datei.LinkUpdatedatei, _dateiverzeichnis + '/' + Path.GetFileName(datei.LinkUpdatedatei));

Also, we have already tried it with an HTTPWebRequest:

foreach (UpdateDatei datei in BezieheUpdatedateipfade())
{
    using (FileStream fileStream = new System.IO.FileStream(_dateiverzeichnis + '/' + Path.GetFileName(datei.LinkUpdatedatei), System.IO.FileMode.OpenOrCreate, System.IO.FileAccess.Write))
    {
        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(datei.LinkUpdatedatei);
        request.Method = WebRequestMethods.Http.Get;
        request.PreAuthenticate = true;
        //ToDo: Credentials aus config auslesen
        request.Credentials = new NetworkCredential(username, password);
        const int BUFFER_SIZE = 16 * 1024;
        {
            using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
            {
                using (var responseStream = response.GetResponseStream())
                {
                    var buffer = new byte[BUFFER_SIZE];
                    int bytesRead;
                    do
                    {
                        bytesRead = responseStream.Read(buffer, 0, BUFFER_SIZE);
                        fileStream.Write(buffer, 0, bytesRead);
                    } while (bytesRead > 0);
                }
            }
            fileStream.Close();
        }
    }
}

The result is always the same and the same files are not downloaded correctly:

Please don't take offense if I forgot or overlooked anything... this is one of my first entries - thanks!