Thanks to pberk for confirming that PagePlus X9 was working fine in Windows 10.
This prompted me to do a lot of housekeeping, deleting unused programs and cleaning up registries -- the usual suspects.
It turned out that I had, at some point, "minimized" a ppb file instead of closing it, and then ended a PagePlus session.
So ever since that time, every ppb file, old or newly-created, was at the bottom of the screen in a minimized condition. It appeared as a blank white screen and clicking it yielded nothing. This time I right-clicked, saw the maximize-minimize button, and, voila!, the ppb book file pops up.
Once I ended the session by properly closing the ppb file and restarted PagePlus, any ppb file opens just fine, and creating a new ppb file creates a new screen just as it should be.
So the happy news is that PagePlus is working fine after all -- and after all this time unsupported by its creators.
Why would any sane person persist in using such an unsupported DTP program? I produce long books with many footnotes (real footnotes, not end-notes), and many chapters, and PagePlus handles these beautifully -- I have done over 1,000 footnotes in one project without anything crashing. I also use the ppb files to organized contents from various books into new books, and the ppb repaginates everything and even lets me impose the styles from one chapter onto all the other chapters. And I like the text editor.
Affinity Publisher has many wonderful features, but they have failed so far to provide footnotes and the kind of chapter and pagination control that I am used to in PagePlus. The alternative, I know, is Adobe InDesign, but I simply do not want to have to start over.
I am glad this Forum is here. Thanks again.