I had a vanilla installation of CRM 2013 running smoothly on Windows Server 2008 R2. I installed Visual Studio 2013, which adds .NET Framework 4.5. I then started getting the error below when browsing to CRM:

HTTP Error 500.19 – Internal Server Error

The requested page cannot be accessed because the related configuration data for the page is invalid.

The actual config error is:

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

I fixed this with the steps below:

1. Change the CRMAppPool in IIS to use .NET Framework 4.0
2. Enable ASP.NET v4.0 for the server by:
   1. Clicking on the server name in IIS
   2. Click ISAPI and CGI Restrictions under the IIS section
   3. Allow ASP.NET v4.0 (x64)