Although it worked well in 2014, I tested blogging with Word again this year and I was surprised that it initially refused to register my WordPress blog account for publishing with a generic error. Most information on the Web related to this type of issue suggested to enable XML-RPC protocol for the blog, but in my case it was already enabled.
The solution that I eventually found was way simpler: I had to use HTTPs instead of HTTP for the URL of the blog. And that was all!
The prompt for WordPress registering is something like: http://<Enter-the-URL-of-the-blog-here>/xmlrpc.php. And it doesn’t work anymore, unless you update it to: https://yourblog.wordpress.com/xmlrpc.php.
Microsoft should really update the Blog Registering dialog window in a next patch (to be delivered as soon as possible) to inject HTTPS protocol by default in that textbox!