When we use a custom UrlRewrite module that does a RewritePath to an existing .aspx file you cannot use the Session Object in that .aspx page by default. The following exception is thrown by IIS7:
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 <configuration>\<system.web>\<httpModules> section in the application configuration.
Readd the Session Module under a different name in the <system.webServer><modules> collection. (must be removed again in web.configs of sub applications)