“Editor has encountered error” on pages/posts

Recently, I’ve noticed that two of my WordPress installations have suddenly stopped being able to either edit or publish new posts or pages. On my host, I have three separate installations on different domains. Two of which have the same problem.

On editing or creating a new page/post, the error “The editor has encountered an unexpected error”. I’ve downloaded the error details below.

I’ve tried disabling all plugins, returning to a standard theme, reinstalling the latest version (via the dashboard) and the same issues is back.

Any help would be most appreciated!

TypeError: Cannot read property ‘mode’ of undefined
at fn (https://mydomain.com/wp-includes/js/dist/edit-post.min.js?ver=99ca111a562584048ddeb818a39c18f6:7:184077)
at je (https://mydomain.com/wp-includes/js/dist/vendor/react-dom.min.js?ver=16.9.0:78:476)
at ph (https://mydomain.com/wp-includes/js/dist/vendor/react-dom.min.js?ver=16.9.0:215:173)
at lh (https://mydomain.com/wp-includes/js/dist/vendor/react-dom.min.js?ver=16.9.0:126:409)
at O (https://mydomain.com/wp-includes/js/dist/vendor/react-dom.min.js?ver=16.9.0:121:71)
at ze (https://mydomain.com/wp-includes/js/dist/vendor/react-dom.min.js?ver=16.9.0:118:14)
at https://mydomain.com/wp-includes/js/dist/vendor/react-dom.min.js?ver=16.9.0:53:49
at unstable_runWithPriority (https://mydomain.com/wp-includes/js/dist/vendor/react.min.js?ver=16.9.0:26:340)
at Ma (https://mydomain.com/wp-includes/js/dist/vendor/react-dom.min.js?ver=16.9.0:52:280)
at mg (https://mydomain.com/wp-includes/js/dist/vendor/react-dom.min.js?ver=16.9.0:52:496)