If a session is opened for editing, and the session is deleted by clicking Delete Session on the Session Manager toolbar, the toolbar does not refresh to a state where just the Open Session Manager button is enabled. All other buttons on the toolbar should be disabled once the session is deleted.
This is caused by an underlying ESRI bug (BUG-000099948) concerning the refresh of toolbars.
The work-around is to refresh ArcMap using F5. Once refreshed, only the Open Session Manager button will be enabled.