OK... I think I solved the issue by updating the jupyterhub.service configuration.
I had to locate the jupyterhub.service file. In my system it was in the folder
sudo gedit /etc/systemd/system/jupyterhub.service
Underneath the existing "Environment=" line, I added another line
After saving the file, I had to re-load the configuration into the system with
sudo systemctl daemon-reload
finally - restart the service:
sudo systemctl restart jupyterhub.service
Now I can run cadabra2 commands in jupyterhub from any computer in my LAN
(I configured a firewall rule to allow http traffic only from LAN IPs).