In the process of installing Orchard tonight, I encountered the following error.

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

The web.config clearly had an empty system.web.extension section.

<system.web.extensions/>

But there was no corresponding section declaration, just like the error said. However, Orchard runs fine on my local machine, and I’ve deployed it to my server before. What could be going on?

It turns out that it’s quite a simple issue, and probably a common problem. If you upgrade a site to .NET 4, and don’t upgrade the application pool to use .NET 4, you will get this error. The machine.config file for .NET 4 declares this section, and earlier version did not have it (unless you manually added it).

In IIS 7, with the site selected, choose Basic Settings in the right pane. This will bring up a dialog that contains the name of your application pool. Then select the application pools tree node and the application pool from the list that shows up. Choose Basic Settings from the right pane, and you can change the framework version.