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I had the same problem after upgrading my system.

In my case, the solution was to remove configurations and data that Jupyter had stored at a couple of places throughout the system.

First, check which paths Jupyter is potentially using:

$> venv/bin/jupyter --paths
config:
    ./venv/etc/jupyter
    /home/<USER>/.jupyter
    /usr/local/etc/jupyter
    /etc/jupyter
data:
    ./venv/share/jupyter
    /home/<USER>/.local/share/jupyter
    /usr/local/share/jupyter
    /usr/share/jupyter
runtime:
    /home/<USER>/.local/share/jupyter/runtime

Some of these paths might not exist, so that you do not need to handle them. Others might have the actual configuration and data that you intend to use (in my case, this is the virtual environment at ./venv/).

Next, (re)move those directories that might contain outdated information, e.g.,

mv /home/<USER>/.jupyter /home/<USER>/.jupyter.bck # keep as a backup
sudo mv /usr/share/jupyter /usr/share/jupyter.bck

Moving the configuration directories resolved some peculiar but harmless warnings that I had. Moving the data directories resolved the 404 issue and allowed me to open my notebooks again. I did not touch the runtime directory.

Please let me know if this approach works for you; we can later update this answer to keep it for later reference.

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