HTTP 500 Errors With Invalid Request Format After Upgrading Site to .NET 4.0

Problem: After upgrading an ASP.NET site to 4.0 you get HTTP 500 errors with exceptions when Javascript is making web service calls: “System.InvalidOperationException: Request format is invalid: application/json; charset=utf-8.” You’ve used IIS Manager to change the application pool to specify .NET 4.0 but the problem persists. Solution: Change to the 4.0 framework folder (C:\Windows\