3262524 - 400 session timed out error with WebGUI calling from Portal

The "400 session timed out" error only happens inside EP(Portal), there is no issue testing WebGUI(SAP GUI for HTML) alone.
Outsdie EP uses old theme tradeshow, which is not support for backend webgui(Basis7.55).

Usually, there is a "500 Internal Server Error (Application Server Error)" before the 400 error checking in the HTTP watch trace.

Calling Webgui in EP

400 session timed out, 500 Internal Server Error, Application Server Error, Portal, Webgui, 400, 500, EPFLP, legacy, not supported, sap_tradeshow, sap_tradeshow_plus, sap_corbu, sap_bluecrystal, custom theme, tradeshow, corbu, bluescrystal , KBA , BC-FES-WGU , SAP GUI for HTML , BC-FES-ITS , SAP Internet Transaction Server , BC-WD-UR , Unified Rendering , EP-PIN-AI , Application Integration , Problem