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
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.