Thanks. I cannot backtrack on my steps, as I did a string of plugin updates all at once, so I’ve no idea where the fault crept in. I updated Yoast to version 22.9 yesterday (after trying some earlier versions, too) but that did not resolve the conflict.
I am not expert enough to diagnose errors/corruption at a deeper level, so regrettably I have had to go to another SEO plugin. I assume that the fault is due to some file corruption on my side, and that Yoast itself is fine.