Further, the problem occurred when I was running the TWENTY THIRTEEN theme, latest version. I tried changing the theme to TWENTY FIFTEEN and the same problem occurred while running other plugins or with them all deactivated except NGG. The problem does not occur when all plugins, including NGG are deactivated.
I have tried to trash and reinstall the NGG – same problem. Could there be settings within the NGG plugin that cause the problem? It seems clear that the problem is between the NGG, together with whatever its settings are, and WordPress 4.8 and whatever its settings are.