I re-activated the plugin and disabled ‘Force SSL for Dashboard’. Visitor access to the site failed with error ‘This webpage has a redirect loop ERR_TOO_MANY_REDIRECTS’.
I logged out as Admin and logged in again: No problem. The Log list showed many foreign login attempts blocked and an occasional host locked out but nothing showing the problem.
I then had to again deactivate the plugin to allow Visitor access.
Thanks, Calmin. You’re right. It was a Theme issue. I still don’t understand why the message only appeared when I wasn’t Admin, but it seems I can dismiss it and continue.