The configuration section 'system.web.extensions' cannot be read because it is missing a section declaration

Try changing the application pool run time from .net 2.0 to .net 4.0

The issue resolved for me after adding the following configuration setting in my webconfig file

<configSections>
    <sectionGroup name="system.web.extensions" type="System.Web.Configuration.SystemWebExtensionsSectionGroup, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
      <sectionGroup name="scripting" type="System.Web.Configuration.ScriptingSectionGroup, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
        <section name="scriptResourceHandler" type="System.Web.Configuration.ScriptingScriptResourceHandlerSection, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false" allowDefinition="MachineToApplication"/>
        <sectionGroup name="webServices" type="System.Web.Configuration.ScriptingWebServicesSectionGroup, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
          <section name="jsonSerialization" type="System.Web.Configuration.ScriptingJsonSerializationSection, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false" allowDefinition="Everywhere"/>
        </sectionGroup>
      </sectionGroup>
    </sectionGroup>
  </configSections>

We can refer this article for more info: http://www.morgantechspace.com/2013/11/The-configuration-section-system-web-extensions-cannot-be-read-because-it-is-missing-a-section-declaration.html

I also had the same problem after upgrading my OS from Windows Server 2003 to Windows Server 2008 R2 , I just changed the application pool settings to .NetFramework 4.0.0 and also changed the classic to Integrated mode ... my problems solved ..