Sure enough, it is a plug-in conflict. The culprit is this old one: https://ww.wp.xz.cn/plugins/scripts-n-styles/
I have deactivated it and can now use your settings menu, and will research a replacement for the script override. Thank you for the suggestion.
I uninstalled and reinstalled the plug-in with no change, making sure to update the site and plug-in just now. Did I really mess it up by not inserting the temporary API?
