I ran into this exact same problem but it seems to be caused by using Session calls in my master page. Whenever I try indexing the site I get the following 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 <configuration>\<system.web>\<httpModules> section in the application configuration."
But I have set the site to use sessionstate in the web.config file. I found this thread and it made me try this approach:
HttpSessionState mySession = HttpContext.Current.Session;
And then try using mySession[""] instead of Session[""] which worked.
My question is why do I have to use HttpContext.Current.Session when I should be able to use Session like I do with every other web site.