Error when attempting to Save As Web Page from Visio 2016

Hi. I am attempting to save a 100 or so page hyperlinked process model as a Web Page using Visio 2016 Version 1803 (Build 9126.2282).

This has worked in the past but after 20 minutes or so now I get:

Save as Web Page: An error occurred while attempting to save the document. Pages created may not be valid.

Nothing is actually generated. This is happening on both our large process files.

The installation is a brand-new laptop.

Any suggestions please?