.net – SSRS problem getting ASP.NET session has expired

when viewing the report in report viewer we get the following error ASP.NET session has expired

we use .NET 2005 ssrs 2005

well we have a report developed and hosted on SSRS server which is same as application server for ASPX applications

in our web application the default.aspx have a report viewer that shows the report

deployed on test server where the link to the application is http://testserver/appname
and report server link is http://testserver/reportserver

every thing is working fine

in production server

we have different sitename for the application which is http://applicationname
in production each application is defferent site

so here we have http://applicationname for the webapplication
and http://servername/reportserver for the report

here the report viewer is not showing the report but giving the following error

ASP.NET session has expired

AspNetSessionExpiredException: ASP.NET session has expired]
Microsoft.Reporting.WebForms.ReportDataOperation..ctor() +866
Microsoft.Reporting.WebForms.HttpHandler.GetHandler() +719
Microsoft.Reporting.WebForms.HttpHandler.ProcessRequest(HttpContext context) +15
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +638
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +177

can i get some help please

update :
tested it on the production as http://productionserver/applicationname
it is working fine (thats when i put the application as a virtual directory under the default site)
but when i put the application as saperate site http://applicationname on the same server i get the error ?!!!