Hi Nadya,
In JupyterHub, there's a "pre_spawn_hook" function which calls XNAT and retrieve the configuration for a user's notebook container (mounts, cpu and mem requirements, etc). The error seems to suggest the configuration JSON returned from XNAT is empty or not valid JSON, which shouldn't be the case.
After starting a notebook from XNAT could you check the response from this API endpoint: /jupyterhub/users/{username}/server/user-options. You can access this in XNAT from: Administer -> Site Administration -> Other -> Miscellaneous -> Development Utilities -> View Swagger Page -> jupyter-hub-api -> endpoint: /jupyterhub/users/{username}/server/user-options
Thanks,
Andy