Website responding with Error message in HttpWebRequest

Question

I have two problems:

 

First, when I am trying to get data from this website it goes well within browser but when I send request using httpwebrequest in C# it responds with this message. In first attempt it may return data but if the send request again it returns this message.

 

Second, when I go to page-2 or click next to go to another page it generates numbers (in end) which is not consistent changes everytime I refresh browser. How can I get these numbers? For example: "=1559040099084" like this.

 

{ "message": "refresh" }

HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(URI);
request.Headers.Add("IfModifiedSince", DateTime.Now.ToString());
request.Method = "GET";
string result = string.Empty;
using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
{
Stream dataStream = response.GetResponseStream();
StreamReader reader = new StreamReader(dataStream);
result = reader.ReadToEnd();
reader.Close();
dataStream.Close();
}
Debug.Write(result);

https://www.rajalacamera.fi/tuotteet/objektiivit

https://www.rajalacamera.fi/alt_api/category/?category_id=29&p=2&product_list_order=position&product_list_dir=asc&_=1559040099084