This error is typically a session problem between the web application (tomcat) and CMS. It can occur during many actions and is often seen when authenticating. Typically if you check the CMC you will see that authentication completed correctly (session created) but the error was received anyway. The most effective way to resolve this is to restart tomcat, or more often stop tomcat, delete the work directory (for the application(s) receiving errors) and then start tomcat again. It is unknown if the issue is actually caused by tomcat (non SAP issue) or an SAP product bug that can be fixed. I don't believe that any fixes have been issues that resolve the error for all occasions.
Regards,
Tim