IPython Notebook error: Error loading notebook

Search for nbsignatures.db. It should be either here:

~/.ipython/profile_default/security/nbsignatures.db

or here:

 ~/Library/Jupyter/nbsignatures.db

The ~ stands for you home directory.

Move this file this to a temporary directory.

Restart your IPython Notebook server.

This won't be the issue for everyone, but I got the below error message because the disk was actually full.

An unknown error occurred while loading this notebook. This version can load notebook formats v4 or earlier. See the server log for details.

remove ~/.ipython/profile_default/history.sqlite

this helps for me.