Security Exception with NAV 4.0 SP3 and Employee Portal 5.0

Hi all,
I installed correctly EP 5.0 on NAV 4.0 SP3, everything seems ok, web part ecc. but when I try to save some data (e.g from customer card) I receive a Security Exception.

[SecurityException: Session timeout occured. Please reopen the current page to proceed.]
Microsoft.Navision.EmployeePortal.WebPartLibrary.DetailPart.Action_Command(Object o, CommandEventArgs e) +2821
System.Web.UI.WebControls.LinkButton.OnCommand(CommandEventArgs e) +105

the event viewer reports one error and one info.

ERROR:
source: Windows SharePoint Services 3
event ID: 6615

INFO:
source: ASP.NET 2.0.50727.0
event ID: 1314

Any Idea?
Thanks in advance