This isn’t a fix for the above; I don’t know why that situation fixed itself, but I got up this morning and the image gallery thumbnails were perfect.
But then I discovered all the thumbnails on the site were now missing. It was a permissions issue. The permissions were changed, allowing “group” full access. When “write” was unchecked for group, things righted themselves.
I don’t know if both are related, but I thought I’d pass on what we discovered.