Thanks. I tried disabling the “3rd-party includes” in my first post above. I have tried it again in case anything has changed, but no difference.
Here is the commentary from the team investigating the problem:
The issue is related to the order in which the jQuery JavaScript library and the JavaScript file from the Memphis Docs plugin you are using are loaded into the page. When we register the JavaScript file in our plugin then we declare any dependencies that the file has, which in our case is jQuery – a very widely used JavaScript framework. We register this in the recommended way:
wp_register_script( 'cffscripts', plugins_url( '/js/cff-scripts'.$cff_min.'.js' , __FILE__ ), array('jquery'), CFFVER, true );
Doing this, loads the jQuery file before any other JavaScript files are loaded into the page (screenshot1). The issue seems to be here that the Memphis Docs plugin doesn’t like that the jQuery library is being loaded before its JavaScript file. When our plugin is deactivated then the jQuery file is loaded further down the page, after the Memphis Docs JavaScript file, and so doesn’t cause this issue (screenshot2). You can see the Memphis Docs JavaScript file immediately above the jQuery library in that screenshot.
I am not sure why the Memphis Docs JavaScript file would cause this issue when jQuery is loaded before it on the page. Ideally, the long term solution would be for that plugin developer to fix that issue as the same problem could happen for any number of other people using that plugin.