Invalid Umbraco configuration data (4.5 on WS2008R2 web install)

After the web installer completes I try and access the new site. /install/default.aspx gives the following 500.19:

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

 171:   <!-- ASPNETAJAX -->
 172:   <system.web.extensions>
 173:     <scripting>

The site is running using the ASP.NET v4.0 application pool in IIS 7.5.

Any thoughts? Google shows plen开发者_高级运维ty of examples of this but no sensible solution(s).

Are you absolutely sure that this specific website is indeed using the .net 4.0 application pool?