Magento 1.9.x – 404 Errors on Prod Server But Not On Local Dev Machine

We’ve updated a Magento installation from 1.9.0.1 to 1.9.3.6. Had to re-code a custom module. Everything is working just fine on our local dev machines, which are a clone of the production server:

Nginx, PHP 5.6.30, MySQL

When we push the revision to production, everything works just fine, the admin panel shows Magento 1.9.3.6 but this one custom module (which modifies the admin customer detail view) throws a 404 error once a form is submitted.

All of this works perfectly fine on our local dev.

We’ve deleted the cache, re-indexed all data and triple checked that the code was transferred correctly. Everything matches the local dev but the custom module throws a 404 error on the prod server.

My questions:

1. Any idea what is going on here ?

2. Since the 404 Magento error page does not state any useful debug information, how can we debug the routing on the prod server?

controller class located at appcodelocalTestCustomCustomerFieldscontrollersAdminhtmlCustomCustomerFieldsIndexController.php:

class Test_CustomCustomerFields_Adminhtml_CustomCustomerFields_IndexController extends Mage_Adminhtml_Controller_Action {}

module config.xml:

...
<admin>
    <routers>
        <adminhtml>
          <args>
              <modules>
                  <customcustomerfields before="Mage_Adminhtml">Test_CustomCustomerFields_Adminhtml</customcustomerfields>
              </modules>
          </args>
        </adminhtml>
    </routers>
</admin>
...