Another component has already added an HTTPSessionState to the Context. Please make sure only one session state module should be registered

Problem:
After changing web application properties from SharePoint central administration site to support “anonymous access”, sites started giving this error message: Another component has already added an HTTPSessionState to the Context. Please make sure only one session state module should be registered

Solution:

Open Web.config file of the SharePoint web application, search for  “SessionStateModule”, make sure only one entry exists under sections such as <httpModules> and <modules>. Comment out any other additional entries!