Am using PayPalCoreSDK , PayPalMerchantSDK.dll and PayPalPermissionsSDK.dll version 1.7.1 for express checkout payment with trail. Everything goes fine in localhost with sandbox credentials but now am moving the same code to live server with the same sandbox credentials am not redirecting to pay pal site the error am getting is Invalid HTTP response The underlying connection was closed: An un Error imageexpected error occurred on a send. please help
Sounds like your new server is not using the TLS 1.2 protocol to send HTTP requests to PayPal, which is now required in the sandbox (and will be on the live servers next year).
I'm not as familiar with .NET as I am PHP, so I can't provide direct info like I typically would when people bring this up, but you just need to make sure your server is sending all requests over the TLS 1.2 protocol and the connection failure should go away.
