Why would I get this error?

Server Error in ‘/’ Application.

Runtime Error

Description: An exception occurred while processing your request. Additionally, another exception occurred while executing the custom error page for the first exception. The request has been terminated.

Is there Database involved in your project ?

You need to open ASP.NET debug mode to see the real error:

https://msdn.microsoft.com/en-us/library/e8z01xdh.aspx