@grace i used your instructions here: O3 Implementer Documentation: Set Up, Configure & Deploy - Projects - OpenMRS Wiki and got exactly the same problem as you did. On looking at the network tab, the call to session was returning a 302. On looking at the general http headers, the Request URL was http://localhost/openmrs/ws/rest/v1/session and for the Response Headers, the Location was http://localhost/openmrs/initialsetup which looked like OpenMRS was still initialising. Therefore because the app tries to make REST calls before the backend is done with the startup process, you will end up breaking the application. I confirmed this by removing the docker volumes and run the docker setup afresh, but this time giving the backend time to properly start before accessing the frontend app, and all turned out well.
This looks like what was reported on the ticket that you assigned to @corneliouzbett [RA-1769] Calling REST API during initialization break Reference Application - OpenMRS Issues