Following an upgrade to some Python libraries, I found that problems with plotting from Matplotlib causing Python to crash were recurring (previously reported here). When called from Excel with pyxll this was also causing instant crashes of Excel.
I eventually found the problem was with the latest version of Freetype, and after downgrading to release 2.10.4, and then upgrading Scipy to the latest version, everything went back to working normally again.
The procedure to downgrade to a specific package using Conda is:
conda install freetype==2.10.4