formgrader page not showing

[t1nux]

I set up jupyterhub-5.2.1 with nbgrader-0.9.3 according to the example One Class, Multiple Graders. All was done using pip in a python-virtualenv.

As one of the instructors, I see the course in the course list under available formgraders, but when I click on it nothing happens.

This does not seem to be the same as #1870 though. Clicking does not do anything directly, but Firefox's debugging consoles gives me:

Uncaught (in promise) Error: Command 'nbgrader:open-formgrader' not registered.

This seems to happen in jlab_core.

Doing the same in chromium, the debugging console gives me:

Uncaught (in promise) Error: Command 'nbgrader:open-formgrader' not registered.
    at g.execute (jlab_core.d25e823c058f4ae79a91.js?v=d25e823c058f4ae79a91:1:1773537)
    at N.o.onclick (603.98d762b1e275ea7b932d.js?v=98d762b1e275ea7b932d:1:21190)

Please let me know if there's anything else I can post.