In the admin section, the visual editor (TinyMCE) has gone blank. Firebug shows a bunch of 404 errors. These files all actually exist but return a 404 when loaded as assets or when I try opening them in my browser.
I tried deactivating all plugins, as well as reinstalling WP 4.6.1 to no avail. I have a dev copy of the site where this issue is not happening.
What could be causing a 404 on an existing file?