Failed to start service - nbgrader with jupyterhub

I try to get nbgrader to run with a jupyterhub 2.3 but get an error (Failed to start service)
It seems to be because a user is not found as it says:
jupyterhub | File “/usr/local/lib/python3.8/dist-packages/jupyterhub/”, line 1487, in user_env
jupyterhub | home = pwd.getpwnam(
jupyterhub | KeyError: “getpwnam(): name not found: ‘grader-course101’”

and earlier tries to list all loaded users (but there are none)
jupyterhub | [D 2023-05-16 12:24:15.898 JupyterHub app:2560] Loaded users:

I currently work with docker spawner and dummy authenticator for some tests.
Might this combination be a problem?
If I remove the nbgrader service in the jupyterhub_config then everything is running. The user grader-course101 then exists (in the container, in the hub-database and even on the server).

Any ideas what to look out for?

Just an update, double check if users really do exist. I used an older docker image (by accident) and there the user was really missing. It is running now.