User Portal and ISA server

Does anyone have experiences with User Portal and Microsoft ISA server? I have installed User Portal and it works fine on the local network, but when I try to work with User Portal through the MS ISA Server I have some problems. Browsing works OK but when I want to update somethnig (for example create new Sales Order) I got message “Access Denied”. I’m sure that the problem is on ISA server so if anyone has some ideas I would be appreciate. Thanks Andrej