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 configuration section group definitions from the application-level Web.config file. These definitions are usually at the top of the application-level Web.config file and can be identified by the configSections element and its children.”

Happy hosting!
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