You know what they say about curiosity
Although a new version of the forum software had been installed recently, it wasn't the issue per se, as I confirmed when I installed the same software at another site (my own). Turns out the new software didn't play well with our version of the server-side php scripting software. Tech Support figured it out, changed the latter, and it resolved the issue.
Meanwhile, apologies that I was in transit (by car) for 2,500 miles, and the fact that I erroneously assumed that a "good" post required a single image (like a screen shot) rather than a matrix of small images. I finally figured this out when I played a few Wordle games.