hi!! are you use sessionstate in your webconfig file
Most webhosting company uses these setting.
but if you getting same error
On the server, go to Administrative Tools – Services:
Service: "ASP.NET State Service" had Startup Type set to Manual, and was not running.
Changed Startup Type to Automatic, and started it. (Right-click on "ASP.NET State Service" – Properties to change settings). Then it worked.