Well after much hand wringing and zero response to my ticket, it turns out the issue lay in the hosted environment and not with my local installs. Since I deal with various setups, it was a challenge to find a workaround. For half of my WP blogs, I had to move off Nginx servers, since the hosting companies effectively blocked calls to xmlrpc at the server level.

For the other hosts running Apache, I was able to implement user-level .htaccess filters which allowed me to use the following lines on the target sites:

<Files xmlrpc.php>
SecFilterInheritance Off
</Files>

OR:

<FilesMatch “xmlrpc\.php$”>
Satisfy Any
Allow from yourdomain.com
Allow from yourdomainip
Deny from all
</FilesMatch>

So if you’ve already troubleshot these areas in your WP install and/or, then give the above a try.

-Checked your wp-config.php for debug mode, charset
-Modify Security Plugins that disable xmlrpc
-Rewrite over your WP core files in case of hacked/corrupted files
-Checked WP database for correct collation/encoding (via phpMyAdmin)
-Updated PHP to take care of broken/disabled PHP xmlrpc extensions (within cPanel)
-If running Varnish cache, disable within cPanel