Page not found error
I restored the database backup that came with your ticket locally and created an article.html template in craft/templates and was able to load the route on the front-end of the site as well as get it to display in Live Preview.

My guess is that either 1) there's something wacky with the permissions on that file that Craft/PHP can't read it or 2) there's a .htaccess rewrite rule in your setup that's interfering with that request.