Some changes to the way JCE scripts are loaded was necessary for Joomla 4 compatability. This change uses the defer attribute on the script tag (used on the core Tinymce scripts in Joomla 4), which delays loading of the scripts.
This is usually not a problem when other javascript referencing functions in these scripts is also delayed (using the "DOMContentLoaded" event or the JQuery "ready" event), but in WidgetKit 1 (and some other extensions), this is not the case, resulting in errors. I doubt Yootheme are going to update such an old extension.
I am looking into ways I can make this work, but it is proving to be quite a difficult task, accomodating so many 3rd party extensions that attempt to load JCE each in their own unique way.
Ryan Demmer
Lead Developer / CEO / CTO
Just because you're not paranoid doesn't mean everybody isn't out to get you.