Hi Kiervin,
What expressway software you are using ?
Did you checked the session timeout value under the system-->administration ? By default its 30 minutes. If you haven't changed it, then it could be possible that the security administrator is inspecting the traffic on port 443 and it is disconnecting the session.
You can run a wireshark on PC and the Expressway to verify what is happening between the two.
Regards,
Alok