"Spawn failed" after removing nbgrader

I installed on my Jupyter hub server nbgrader. I had problems with nbgrader and so I deinstalled it with

sudo -E conda remove nbgrader

Now I can’t start the Jupyter server for any user. There’s an error

Spawn failed: Server at didn't respond in 30 seconds

What can I do? How can I purge Jupyterhub and reinstall it?