Error in XML document (1, 1) while deserializing from web, no error when deserializing local file

When I download the XML from this URL and deserialize from hard disk, there's no error. When I'm deserialize from the URL with Streamreader and Webrequest or XmlDocument.Load or XmlTextReader I got every time the error "Data at the root level is invalid. Line 1, position 1 "

I've tried already a lot of possible several ways to read the online XML. I also tried to force the encoding to ISO-8859-1 as given in the XML but without result.

string path = "http://publications.elia.be/Publications/Publications/InternetImbalance.v1.svc/GetImbalanceMeasuresByTime";
var col = new Model.EliaImbalanceMinutes.AllData();

try
{
    using (TextReader reader = new StreamReader(WebRequest.Create(path).GetResponse().GetResponseStream(), Encoding.GetEncoding("ISO-8859-1")))
    {
        XmlSerializer serializer = new XmlSerializer(typeof(Model.EliaImbalanceMinutes.AllData));
        col = (Model.EliaImbalanceMinutes.AllData)serializer.Deserialize(reader);
    }
}
catch (Exception ex)
{
    MessageBox.Show(ex.Message);
}

It gives me this error:

System.InvalidOperationException HResult=0x80131509 Message=Het XML-document (1, 1) bevat een fout.