People get this error a lot when upgrading an ASP.NET 2.0 application to ASP.NET 4.0. Here is a link to common ASP.NET 4 Breaking Changes: http://www.asp.net/learn/whitepapers/aspnet4/breaking-changes#0.1__Toc256770149 And here is the section that applies to this error: “The workaround for the second scenario is to delete or comment out all the system.web.extensions configuration section definitions and […]