IsNewSession problem

SYSTEM:
WINDOWS SERVER 2003 R2
DYNAMICS NAV 4.0 SP3
DYNAMICS NAV EMPLOYEE PORTAL 5.0
WINDOWS SHAREPOINT SERVICES 3

SYMPTOMS:
When I click on Save, Create New or Delete from Customer Card Web Part
(installed with Employee Portal)

ERROR:
[SecurityException: Session timeout occured. Please reopen the current
page to proceed.]
Microsoft.Navision.EmployeePortal.WebPartLibrary.DetailPart.Action_Command(Objec
t
o, CommandEventArgs e) +2778
System.Web.UI.WebControls.LinkButton.OnCommand(CommandEventArgs e)
+105
System.Web.UI.WebControls.LinkButton.RaisePostBackEvent(String
eventArgument) +132
System.Web.UI.WebControls.LinkButton.System.Web.UI.IPostBackEventHandler.RaisePo
stBackEvent(String
eventArgument) +7
System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler
sourceControl, String eventArgument) +11
System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
+177
System.Web.UI.Page.ProcessRequestMain(Boolean
includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
+7350
System.Web.UI.Page.ProcessRequest(Boolean
includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
+213
System.Web.UI.Page.ProcessRequest() +86
System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)
+18
System.Web.UI.Page.ProcessRequest(HttpContext context) +49
ASP._catalogs_masterpage_defaultlayout_aspx.ProcessRequest(HttpContext
context) +4
Microsoft.SharePoint.Publishing.TemplateRedirectionPage.ProcessRequest(HttpConte
xt
context) +153
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Ex
ecute()
+358
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean&
completedSynchronously) +64

CODE:
public sealed class DetailPart : BasePart
...
internal override void Action_Command(object o, CommandEventArgs e)
...
if (this.Context.Session.IsNewSession)
{
throw new
SecurityException(ErrorLibrary.GetError(ErrorNumber.UpdateSessionTimeout,
false));
}
...

We have some problem with IsNewSession: is always true.