We’re using Classic Editor and Add-on on our Genesis sites and ran into the same issues described here.
First, thanks to everybody for your helpful contributions.
The trick of uninstalling Classic Editor and Classic Editor Add-on got rid of the first error message. We then reinstalled but now have had to deactive Classic Editor add-on to remove string of unwanted text (error code) across the top of page.
For now, discontinuing ALL site upgrades to WP 5.3 until we learn this is resolved.