Thanks for the ideas!
All check out as okay, but the plug-in check, a long task which I will begin now.
I can’t help thinking that I may have changed a setting somewhere. I have added no new plugins since the error appeared (though some have updated with the new WP), and all my plugins are reliable/stable and support for the likely culprits reports no similar errors.
More basically, is there any normal WP setting which may do this?
