Invalid HTTP response the underlying connection was closed

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ASP.NET

C#3.5

PayPal

, +

i facing some problem in Paypal integration,

i find error of this block mainly last line of code on debug time .

ReceiverList sl = new ReceiverList();
           sl.receiver.Add(rec);
           PayRequest req = new PayRequest(new RequestEnvelope("en_US"), "PAY",
                              cancelURl, "USD",
                              sl, returnURL);
           req.reverseAllParallelPaymentsOnError = true;
           AdaptivePaymentsService service = new AdaptivePaymentsService();

           PayResponse resp = null;

           resp = service.Pay(req);

error is :

Invalid HTTP response The underlying connection was closed: An unexpected error occurred on a send.

What I have tried:

Update API Setting on Paypal Account etc ..