Issues and Solutions for Sharepoint 2010 - Enable Session State Issue

This issue arises due to missing or removing session element. For that just verify whether the "enableSessionstate" is set to "true" and add the following line in the httpmodules in web.config file.

1) <pages enableSessionState="true" enableViewState="true" enableViewStateMac="true"/>

2) <add name="Session" type="System.Web.SessionState.SessionStateModule" />

If this doesn't work check whether "Session" is removed somewhere like this,

<remove name="Session" />.

If so add the 2nd line again after that. 

For a detailed article visit this blog.