OK, a few possibilities:
There may be corrupt cache entries. To clear the cache in Office 2019 / Office 365:
However the issue came back. Even doing a "Reset this PC" didn't help, when I reinstalled Office 365 with the default installer, the issue came back.
The finial fix for me was to install Office 365 32-bit (the default is now 64 bit), and the issue is fixed for now.
