I posted this on my other thread, but I thought I’d post this here too.
The problem for me was my web server. The 404 error code was being generated by an actual 406 error code. 406 error codes are usually generated by mod_security, a module that exists on many web servers server-software (apache). This module filters all post requests and other things according to a set of rules. So, as soon as I’d post something to the wp-admin directory that wasn’t whitelisted it would fail. But it wasn’t a 404, but a 406. I determined this by temporarily disabling my index.php file and cUrling the server for an unadulterated result (the index.php page wasn’t able to handle the 404).
For some people the solution is to put a mod_security exception into an .htaccess file either at the top directory, or the wp-admin directory. I tried that, in several forms, but it either didn’t work, or gave me a 500 internal server error. Unfortunately this is not really the best option anyway as it can expose your website to troublesome atack-traffic.
Finall the solution for me was to contact my hosting company through a ticket aboud their mod_security set up. Since they were interested in keeping my business they modified the mod_security rules to allow this type of traffic (I don’t know the details) and now my proofreading post to wp-admin/admin-ajax.php?action=proxy_atd&url=/checkDocument works beautifully across my whole server.
Hope this helps the next guy, I spent all day on this.
Asher