If you’ve ever seen this error, you probably pulled your hair out (if you still have some) and can’t figure out why IIS won’t just serve up your page. What is this madness about “There is a duplicate ‘system.web.extensions/scripting/scriptResourceHandler’ section defined” nonsense? Well, this is one of those things that is super simple to fix (most times) but can cost you a day of development to figure out what is wrong.

 

 

 

 

 

 

 

 

 

 

 

All you have to do is change the Application Pool’s .Net Framework version from 4.0 to 2.0. Caveat: Only do this if you need to use 2.0 (which also includes 3.5, since 3.5 is 2.0 extended, essentially), if you need 4.0 you normally won’t see this error, as it has to do with the web.config being the wrong version for the configured IIS version. Here’s how to fix it:

Open IIS Manager and Click “Application Pools” and find the one that matches your website. ACME Co, for example. Double-click it and you’ll get the Pop-up window. Change the .Net Framework version to 2.0.50727. It should look like this: