Since the latest update of wordpress, 5.0.2, the Gutenberg update, wordpress sites can no longer have the wordpress domain name different from the site domain name in the general settings – even when those two domain names point to the same server.
WordPress cannot always be posted on your main server
Example. You have a wordpress installation at myExtraDomain.com on ip address 188.8.131.52. This server is a good place to create a wordpress site. However, your main site is at myMainDomain.com on a different server. MyMainDomain.com is a not a good place to host wordpress, but is ideal for all my other functionality. However, I want to use blog.myMainDomain.com to access my wordpress blog, so that the second level domain will be consistent for all my links.
What no longer works
Your old set-up was:
wordpress url: myExtraDomain.com/blog site url: blog.myMainDomain.com
This seems the intuitive way to set this up and it worked prior to wordpress update 5.0.2. However, now your site will no longer update or publish, giving a CORS (Cross Origin Resource Sharing) error, even though both myExtradomain.com and blog.myMainDomain.com are actually pointing to the same server.
What works now
The fix is just to change the set-up to:
wordpress url: blog.myMainDomain.com/blog site url: blog.myMainDomain.com
You still need an A Record on myMainDomain.com pointing to 184.108.40.206. Now the CORS error is gone.
Although both set-ups should work, the second is the only set-up that now works. I hope this helps someone else who has found themself in this situation.
Thanks to Otto on WordPress http://ottopress.com/ for help in troubleshooting this issue.