Posted by: lluppes | February 23, 2011

DotNet4 App Won’t Work Under IIS7

I recently upgraded a server to Windows Server 2008 and IIS7.  The root web application is configured to run DotNet2.  Under IIS6, I had a DotNet4 application as a child web and it ran just fine.  Once I upgraded the server to IIS7, I started having issues.  The DotNet4 web refused to even load, getting an 500.19 error:
“There is a duplicate ‘system.web.extensions/scripting/scriptResourceHandler’ section defined”.

Of course, you can’t see that error on the client, so you have to log on to the server to see this screen:


At that point, I looked and looked, but there was no <sectiongroup> section in the DotNet4 web.config.  Some searching pointed me to the ASP.NET 4 Breaking Changes document at

That document suggested that we needed to move the <configSections> out of the root DotNet2 web.config
back to the framework folder config files, which are located in:

Once I did that – everything worked fine.  Now my DotNet4 application works fine under a DotNet2 parent root, just like it used to when I was running IIS6.  Very strange.  Good tip to remember for future use.

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