object reference not set to an instance of an object

i have more 50 users using the application and only one user when she tries to open the task is getting the error but other users when accessing similar task are able to work without getting any errors:
can an expect tell me what does this error mean:

"[NullReferenceException: Object reference not set to an instance of an object.]
   GFC.Security.SecurityIdentity.get_IsAuthenticated() +3
   AMS2.Global.Session_Start(Object sender, EventArgs e) +303
   System.Web.SessionState.SessionStateModule.RaiseOnStart(EventArgs e) +132
   System.Web.SessionState.SessionStateModule.CompleteAcquireState() +514
   System.Web.SessionState.SessionStateModule.BeginAcquireState(Object source, EventArgs e, AsyncCallback cb, Object extraData) +691
   System.Web.AsyncEventExecutionStep.System.Web.HttpApplication+IExecutionStep.Execute() +66
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +173"