Just two days ago, I decided to upgrade to IIS 7 so I'll be able to use .NET 4.0. Then I have installed Umbraco4.7, however I got the following error:
"Default Document
There was an error while performing this operation.
Details:
Line number:172
Error : The configuration section'system.web.extensions' cannot
be read because it is missing a declaration"
Please help me figure out how to fix it.
Problem Occur After Installing Umbraco4.7
Check the Application pool settings.
1. Note the Application pool what your site is using from IIS manager
2. Select the Application pool that you are using by expanding Application pool's
3. Open properties and change the following settings .Net Version to .net version 4 and Managed pipeline to integrated mode.
You need to change even AJAX .NET 4 setting if your site is using it.
If your site is using AJAX of other controls rather than of ASP.NET you need to change those by editing in you web.config file.
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This is how you need to change the settings. Please see the below image which will help you in doing the configuration of web.config file  which all you need to change.
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Please let me know if you need any more help.
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