Viewer doesn't load old files?

I wanted to use the Calibre Viewer to look at files I added to my library a while ago (says 2017). When I click on the book file to load it, the viewer window comes up as expected, but remains blank with 0% loaded in the bottom. It seems to happen with files I built in Word, then imported to my library as EPUB; files I loaded directly as MOBI open just fine.

FWIW, the program does load the file onto my Kindle correctly; it just doesn't show up in the Viewer.

I'm running Windows 10 Home, and just upgraded Calibre to 7.15.0.

Hope someone can help!