An update--after importing old photos one event at a time, and having no problems up to a certain point, I created a new library and imported some brand-new photos. After editing 15 photos (just normal pictures, no videos), the error occurred again, even though the new photos were in a separate library.
What else can be done? At this point, I'm considering looking into a new photo-editing program. I can't believe after all this troubleshooting, the problem still isn't fixed!