There are two (or more) systems in the scenario and first system generates responses, which include protected content from the other systems that requires authentication (e.g. Federated Portal).
500 Internal Server Error is returned and the following exception can be found in the traces: “com.sap.engine.services.servlet_jsp.server.exceptions.WebSecurityException: Security session not found. User is not logged in. Http Session is [XXXX]”.