I recently tried to post a blog entry after upgrading WordPress from v4 to v5 and discovered that I couldn’t save a draft or publish it. The diagnostic in the console wasn’t all that helpful to me. Entries such as

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://www.mrp.net/blog/index.php?rest_route=%2Fwp%2Fv2%2Fposts%2F255&_locale=user. (Reason: missing token ‘x-http-method-override’ in CORS header ‘Access-Control-Allow-Headers’ from CORS preflight channel).
Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://www.mrp.net/blog/index.php?rest_route=%2Fwp%2Fv2%2Fposts%2F255&_locale=user. (Reason: CORS request did not succeed).

This came as quite a surprise and challenged my ability to work out how to fix it. Fortunately I eventually discovered mention of a change of editor (which I had noticed) and that the old one was available so I installed it and my problem was solved. I still don’t understand what is going on with CORS but I have a viable workaround.