Is your .htaccess file inside the ACP folder like this or similar?
Code:
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{HTTP_HOST} ^www\.(.*) [NC]
RewriteRule ^ http://%1%{REQUEST_URI} [L,R=301]
RewriteCond %{REQUEST_URI} !\.(js|css|gif|jpg|png)$ [NC]
RewriteRule ^(.*)$ index.php [QSA,L]
Are the Read-Write-Execute permissions of those files set to 644?
Besides that and the .htaccess, I don't know what's causing that, HoF might be more useful x.x