Session state can only be used when enableSessionState is set to true, either in a configuration file or in the Page directive- SharePoint2010

Complete Error Message

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.

Before you start your troubleshooting, make sure that the State Service service application is running. How you will check this?

The security validation for this page has timed out. Click Back in your Web browser, refresh the page, and try your operation again

Problem Description
The security validation for this page has timed out" error message when a user submits data to Windows SharePoint Services

Error Message
The security validation for this page has timed out. Click back in your Web browser, refresh the page, and try your operation again.

If you are facing this error message then Microsoft has already published an article on this which is very useful and resolves your issues. Million thanks to Microsoft for publishing this detail information along with root cause.
Thanks to MS

Resolution
http://support.microsoft.com/kb/888828

Content databases contain orphaned items

Problem Description
The SharePoint Health Analyzer detected an error. 

Content databases contain orphaned items.