IIS8 Website web.config HTTP Error 500.19 - Internal Server Error "There is a duplicate 'system.web.extensions/scripting/scriptResourceHandler' section defined"

Today i copied a working website from IIS7 to IIS8. I created a new AppPool and WebApplication for it. The ApplicationPool was configured to use .NET4.0

Testing the Website, i received the following error:

HTTP Error 500.19 - Internal Server Error
The requested page cannot be accessed because the related configuration data for the page is invalid.

There is a duplicate 'system.web.extensions/scripting/scriptResourceHandler'

Config Source:
4:
5:

6:

How-i-fixed-it:
I found some useful hints here: http://www.devexpress.com/Support/Center/Question/Details/Q458935
and here:
http://www.britishdeveloper.co.uk/2010/03/there-is-duplicate-systemwebextensionss.html

I upgraded the WebProject to VisualStudio 2012. In the project properties i set the target Framework to .NET 4.5 and did a clean build.
I copied the new build to the WebApplication physical directory and it worked again.