Search within TinyMCE add link isn't working (XHR returns 404)

I'm having an issue where when I try to use the search function to add a link in modx I get a 404 error. Here is an example of the error when I visit the URL that the AJAX request calls directly (URL: http://www.scorchsoft.com/assets/components/tinymce/jscripts/tiny_mce/plugins/modxlink/search.php):


If I edit that file and echo ' --- ' then I can see some output:


This tells me that this is not a .htaccess issue. Also, folder permissions are set correctly as per recommendation in this post:
https://forums.modx.com/index.php?topic=59330.0
Though this article is 2 years old and most likely no-longer relevant.

Edit: It is also worth noting that I'm on 2.2.8 and have recently upgraded to this. Re-installing and re-upgrading doesn't fix.
Edit: Also there are no entries in the MODX error log that relate to this issue / from today. [ed. note: scorchsoft last edited this post 13 years, 2 months ago.]

It could be a permission issue. Check the permission on the folder: /assets/components/tinymce/.
For troubleshooting you could set the permissions temporarily on the components folder and subfolders/files to 777.

I've already tried that unfortunately.