The warning message you got doesn’t seems to have a link with the Guest Plugin in my opinion.
I recently tried to launch backstage in a containerized environnement and got the same error message when trying to login with the Guest sign in card.
For me the issue was the CORS that didn’t allowed the front end process to call the backend.
If you look in the console while clicking « Sign-in » you might see a explicit error on CORS if you are in the same situation.
To fix this, make sure you set CORS accordingly in the app-config.yaml (backend.cors.origin), by putting the url of the front end process in it.