The easiest thing is to update to version 2.3.5 or higher. It solves a redirecting issue. If you want to learn more about the error, read on:
A customer suddenly was not able to add properties to a global schema. The issue was that jQuery sent a POST-request to a WordPress REST-API-URL that had no trailing slash. However the webserver redirected every request to the URL including the trailing slash. And the redirect was made with GET (instead of POST). So all form parameters got lost and the REST-API responded with an error (because it couldn’t find the form parameters).
So for example, the POST request to the following
resulted in a GET-Request to
(notice the trailing slash). That was – of course – wrong. And to fix this I added the trailing slash in every request jQuery made so avoid the redirects.