Start Nintex K2 workflow error "The underlying connection was closed: Could not establish..."

I have a workflow (K2 Five 5.3 with FixPack 03).  I have an application where user can start a K2 workflow).  After applying the Fix Pack, I see this error

"The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel."

We verify that K2 server allows TLS1.2

Here is my code

var URL = "https://s....";

HttpWebRequest request = WebRequest.Create(URL) as HttpWebRequest;

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12 | SecurityProtocolType.Tls | (SecurityProtocolType)768 | (SecurityProtocolType)3072;
ServicePointManager.Expect100Continue = true;
ServicePointManager.DefaultConnectionLimit = 9999;
// get response
var response = request.GetResponse();

....

Thanks in advance.