I broke jupyter (again)

edit

I was trying to install some dark themes, so I ran

$ sage --pip install jupyterthemes

followed by

$ sage --pip install --upgrade jupyterthemes

as suggested here.

Then, when I launch

$ sage -n jupyter

the server seems to open correctly, but when trying to open an .ìpynb file, I get

500: internet server error

Sage itself is running, from the command line everything is o.k. More than that, running

$ sage -n jupyterlab

gives no problem at all: I can still run my notebooks.

Running sage 9.1 on Ubuntu 18.04. At some point I installed hide_code as per this thread. Perhaps my problem comes from some broken dependencies?