“/lab” (JupyterLab) not found on a working JupyterHub installation (“tree” working normally) – JupyterHub

Hello, I manage our laboratory server where we have JupyterHub working with traditional notebooks under Apache proxy server under the “Jupyter” folder on a Ubuntu 20.04 server.
It works great.
I am now trying to add JupyterLab instead (or en plus) of “classic” notebooks.
I have added it using pip (as for the rest of the system) and restarted jupyterhub service (sudo systemctl restart jupyterhub) but when I go (manually or by setting c.Spawner.default_url = "/lab" in jupyterhub_config.py) to https://[server]/jupyter/user/[my user]/lab I get a 403 not found error.

What could I have missed ?

In a forum I got to try “# jupyter lab build”, but when I try Iget an error that it requires nodejs >=12.0.0 while my apt-get says “nodejs is already the newest version (10.19.0~dfsg-3ubuntu1)”.
Ok, I have added the repos and installed node 19, “jupyter lab build” now works, but still 403 error on “/lab” :-/

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