Since Chromium has become a snap package, it cannot in a standard install access just any files.
When launching Jupyter Notebook (executing jupyter notebook in a terminal), this directs the default browser to open file:///home/user/.local/share/jupyter/....html, which of course fails with Chromium for the reason stated above. This is not a huge deal thanks to Jupyter offering access via http as well: http://localhost:8888/?token=....
What I would like is for Jupyter Notebook to forward the localhost route to Chromium automatically. I cannot find an appropriate setting in ~/.jupyter/jupyter_notebook_config.py. The best I can do is set c.NotebookApp.open_browser = False so that I do not have to close the extra tab in Chromium.
Note that, because of reasons, I need to use Jupyter Notebook instead of Juptyer Lab, for now.