So I downloaded the newer Yoast Breadcrumbs file at the link above. To be cautious, I didn’t overwrite anything. I copied the text of yoast_posts.php to a text editor for safe-keeping, and pasted the text of the yoast breadcrumbs in using the editor in WP dashboard. (I know, I didn’t notice they are supposed to be different files till afterwards, I’m learning on the fly.)
When that didn’t fix my problem, I pasted the original yoast_posts text back into the editor and clicked update. Theoretically, everything is now as it was before my little experiment.
However, if I refresh my site or my dashboard now, or click any link in either, I get a totally white blank page. BAD THING.