The issue is caused by some javascript in Virtuemart, which attempts to create a new editor instance, before all javascript (including JCE javascript) has loaded on the page, ie:
<script type="text/javascript">// Creates a new editor instance
tinymce.execCommand("mceAddControl",true,"field-15-customfield_value")
</script>
This script is in
administrator/components/com_virtuemart/models/customfields.php on line
788
The reason this happens with JCE Pro 2.9.12+ and not JCE Pro 2.9.11, is that the way the javascript is loaded in 2.9.12+ was changed for Joomla 4 compatability. Nevertheless, Virtuemart should not try an execute a javascript function until it is sure that javascript function has actually loaded!
Until Virtuemart fixes this (you might want to inform them of the issue), you will need to use JCE Pro 2.9.11
Not sure why this worked in my tests though...
Ryan Demmer
Lead Developer / CEO / CTO
Just because you're not paranoid doesn't mean everybody isn't out to get you.