It looks like at some point the (not packaged in debian) module https://github.com/Jupyter-contrib/jupyter_nbextensions_configurator was installed and added to the nbextension config.
It's not obvious that this can be related to ipywidgets; it might be related to the old method of setting the nbextension config in /etc/jupyter/nbconfig/notebook.json, which might conflict after upgrade with user-level config? `jupyter nbextension list` should provide more information.
