This was discovered using preside through command box.
1) create a sub site in preside - same domain, different directory as the main site
2) Edit the homepage for that new site
3) the ckeditor doesn't display the widget/asset custom buttons
Viewing the page html, It appears to be trying to include this file:
Instead of this one:
commandbox 3.5, windows 10, preside 10.7.27 and earlier versions