Custom Fields Editor Not Loading

Hi,

I've inherited maintenance of a WordPress site which uses Metabox for Custom Post Types and Custom Fields across a range of Post Types.

I've encountered an issue editing the Custom Fields for one particular set of Post Types; the editor never completes loading. When I look at the console in Firefox I'm told that there is a JS error in react-dom.min.js : s.map is not a function, but the stack trace goes back to /wp-content/plugins/meta-box-builder/assets/js/app.js?ver=4.0.5:1 so it's obviously some issue with MetaBox.

My hunch is that s.map is being called expecting s to be an object with the appropriate proptotype, but that's not been defined, so metabox is likely expecting something in the saved config in WordPress which isn't there; but as that's all serialized data in a field I really don't want to start messing around with it.

I was going to post the JSON from the related wp_posts.post_content for the Custom Field Types but it contains information that would reveal my clients website so I'm reluctant to do that on a public forum.

I've had a dig around and can't seem to find an explanation for this - though I could have missed it. Any pointers as to where to look or what further details you may need from me to resolve this?

The Custom Fields set is applied across a wide range of Post Types on this site so it's quite a problem not being able to get in an edit. Other Custom Field types are loading

Many thanks for your help.