If we are correct then you are getting the session id by using user/session service and the session which you are using is in invalidated. If you want to verify the session of the user on the server, then please use this method(http://api.shephertz.com/app42-docs/session-management-service/#get-session-create
) which will give the session id which exists on the backend,
Please have a look into it and let me know if you still face any issue.
Note: This error comes only when the session id is invalidated. By default, there is no expiry time on the server for session id.