You cannot call the Server.MapPath method in the Session_OnEnd script. By default, Session_OnEnd runs as the Anonymous User, as defined for the application. In the event that there isn't an Anonymous user, or the Logon for the Anonymous user fails, the OnEnd function will not be called, and an event will be logged.
Client: Requires Windows XP Professional, Windows 2000 Professional, or Windows NT Workstation 4.0.
Server: Requires Windows Server 2003, Windows 2000 Server, or Windows NT Server 4.0.