admin-ajax.php failing on upload because of plugins_loaded

This is a complex & baffling problem, probably with a simple solution.

One of my client’s sites was written by a 3rd party agency, and has worked fine (mostly) since I’ve taken it over. Recently, one part of a custom-plugin has stopped working, it’s meant to allow the admin to upload images/videos, to be saved as child posts.

I’ve figured out where the problem is, and tried debugging for hours with no success.

– the upload form is generated by a call to /wp-admin/admin-ajax.php
– on submit, the form POSTS a file and a few fields to the same URL /wp-admin/admin-ajax.php, which now returns 302 moved temporarily
– basic debug-logging shows that the problem is in wp-settings, it gets to do_action( ‘plugins_loaded’ ); and then goes wrong.
– I’ve tried disabling all non-essential plugins (leaving this custom-plugin, BuddyPress, and ZEND), with no joy

I don’t know the best way of debugging further, to try to figure out exactly which functions are failing in plugins_loaded(), or why WP is deciding to return a 302.

Any insight on the above paragraph would be GREATLY appreciated, this is rather urgent too ??