Server cannot modify cookies after HTTP headers have been sent

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

HttpException (0x80004005): Server cannot modify cookies after HTTP headers have been sent.]

   System.Web.HttpCookieCollection.Add(HttpCookie cookie) +99

   DotNetNuke.Entities.Users.UserOnlineController.TrackAnonymousUser(HttpContext context) +825

   DotNetNuke.Entities.Users.UserOnlineController.TrackUsers() +148

   DotNetNuke.HttpModules.UsersOnline.UsersOnlineModule.OnAuthorizeRequest(Object s, EventArgs e) +285

   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +92

   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +64

Merci de votre aide!!!!!!!!!!!