Web.config Error

Try removing the section before the second definition all within the Subproject web.config.

See: http://forums.iis.net/t/1155685.aspx

You can remove predefined section or section group by using element in the sub sites' web config file. It looks like this:

<configuration>
 <configSections>
  <remove name="system.web.extensions "/>
  <!-- Add your new section or section group -->
  </configSections>
</configuration>

For more information,

please refer to: <remove> Element for <configSections> http://msdn.microsoft.com/en-us/library/aa309404(VS.71).aspx

I had these exact symptoms and my issue was similar to Peter's. Was setting up an existing project on a new server. My project referenced the IIS7 URL Rewriting module, but that hadn't been installed yet on the new server. Installing it fixed my issue.

You can use the Microsoft Web Platform Installer to install it. Execute it, select Products, in the left menu select Server and find URL Rewrite in the list and install it.

Or you can download it here.