ASP.NET

I am using paypal sandbox to doing testing, however after click the continue button that i wish to complete the transaction, but the page is keep loading. So i click to go back on the browser, and then click to go forward again, then it will only display "You've already completed your purchase. Please click Close to continue to the merchant." My problem is i want it auto return me the success page, but not click the back and forward to view the return to merchant. thanks you very much and appreciate your answer.
The page keep loading URL
https://www.sandbox.paypal.com/webapps/checkout/webflow/sparta/expresscheckoutvalidatedataflow?execution=e8s2[^]

Dim baseDire As String = Server.MapPath("~/MyFILES/")
If IO.Directory.Exists(baseDire & Request("Req_ID").ToString()) = False Then
               IO.Directory.CreateDirectory(baseDire & Request("Req_ID").ToString())
           End If

For Each file As UploadedFile In UploadPropDoc.UploadedFiles
               Dim bytes(file.ContentLength - 1) As Byte
               file.InputStream.Read(bytes, 0, file.ContentLength)
                'Save the File to the disk
               file.SaveAs(baseDire + file.GetName())
           Next

file.SaveAs(baseDire + file.GetName())