WordPress Theme Editor not working – Error 404 page

Summary:

1. Cannot edit theme files for a custom theme as when I hit Update File it redirects me to a 404 Error page on my blog instead of staying in the WP Admin area.

2. The theme files that I’m having this issue with are: front-page.php, functions.php, theme-defaults.php. The other theme files style.css and page_landing.php are updating fine, and when I hit Update File, it says file edited successfully and I am in WP Admin.
All the other themes I have are working just fine.

3. I’ve tried deleted the entire website, re-making it, re-installing the wordpress script, and genesis + child theme, and trying to re-edit the files but it still won’t work, and still redirects me to the same 404 page on the SAME theme files.

How can I fix this?