The famous session state error:

Session state can only be used when enableSessionState is set to true, either in a configuration file or in the Page directive. Please also make sure that System.Web.SessionStateModule or a custom session state module is included in the \\ section in the application configuration.

can be fixed most of the time by using the well-known 3 steps, found here. If that *still* does not fix it, I found that the IIS7 setting runAllManagedModulesForAllRequests=”true” on system.webServer / modules makes the error go away.