Access Denied After Updating Web.Config to 4.5.1 Framework

Windows Server 2008 R2.  IIS 7. Visual Studio 2012.
I modified my web.config file to upgrade from .NET Framework 3.5 to 4.5.1 per the manual instructions at http://msdn.microsoft.com/en-us/library/vstudio/dd483478%28v=vs.100%29.aspx.  No other files in my web site changed other than the web.config.  The web site builds (compiles) fine in VS, with no errors.  Before the change in framework, I could select the default.aspx file, run it in debug (on my local computer) and the site came up fine.  Now when I select default.aspx and try to open in the debugger (IE 11), I get the following error page:
Server Error in '/' Application.
Access is denied.            &nb ...