I am using my local VS2013 environment for development and the PayPal Sandbox settings for the plugin.
I change the code as suggested. But I still get the same error (this is in my local environment using VS2013 with sandbox settings). I am running site as http://localhost:portName in http mode (IIS Express)
Below is the notification from Pay Pal in this regards
I have installed VeriSign certificate but the integration is failing. I am searching around it and keep you posted but let us know if you get through this issue with possible steps to resolve this issue.