I am using PayPalStandard plugin of NopCommerce. When I placed the order & make payment with paypalstandard plugin after successful payment on paypal, it redirects to mercha
The answer that worked for us was listed on the PayPal blog post, Upcoming Security Changes Notice. There are a number of things listed in the post, but the one thing which we did, and that worked, was PayPal SDK Updates. We updated using NuGet and everything started working again.
I had the same issue connecting to sandbox(nvp), everything was fine then yesterday the message "The request was aborted: Could not create SSL/TLS secure channel." appeared.
I believe PayPal updated their endpoints on 19/20 January 2016 to use TSL 1.2 and HTTP 1.1.
To resolve this, for .NET 4.5 and above add the following line of code before calling WebRequest.Create().
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;