OK, so working on the premise that the problem was caused by a plug-in, as it was over a year ago, I wasn't sure which one to disable. There are now several that look similar to the one that caused the problem last year. So I checked off "Show only user installed plugins", and the only similar one listed was:

"Kindle 2, 3, 4, Touch, PaperWhite, Voyage Device Interface Paperwhite thumbnail FIX MOD (1.0.0) by John Schmber, ElGabo COmmunicate with the Kindle 2.3.4.Touch/PaperWhite/Voyage eBook reader. Modify the thumbnail size for Paperwhite"

With that one disabled, and the following one left enabled, the thumbnail is now good:

"Kindle 2/3/4/Touch/PaperWhite/Voyage Device Interface (1.0.0) by John Schember Communicate with the Kindle 2/3/4/Touch/PaperWhite/Voyage eBook reader." (this was left enabled).

I didn't specifically install that first one, so I'm not sure why it gets listed under "user installed plugins", but in any case, that fixed the problem.