I can't reproduce:
Not sure why it is pointing on a file called CATEGORIES.PHP, when the right one is CATEGORY.PHP. I should have changed that in some point, so it's a FALSE positive there. Anyway, if you have FTP access to your server, access the applications / links / api / and see if you have category.php there. If you have it AND also have categories.php, delete categories.php and you're fine.