Answered Code: 200 OK {"success":false,"message":"Permission denied!","total":0,"data":[],"object":[]}

Такая же ошибка возникала, когда в настройках nginx были следующие строки

limit_zone slimits $binary_remote_addr 5m;
limit_conn slimits 5;

Т.е. modx загружал свои ресурсы, но в итоге превышал допустимые 5 одновременных запросов и вместо части ресурсов получал код 503.
Для нормальной работы modx оказалось достаточным:

limit_zone slimits $binary_remote_addr 10m; // на всякий случай
limit_conn slimits 10;

Code: 200 OK
{"success":false,"message":"Access denied.","total":0,"data":[],"object":{"code":401}}

<?php
$corePath = $modx->getOption('core_path',null,MODX_CORE_PATH).'components/customTVs/';
switch ($modx->event->name) {
    case 'OnTVInputRenderList':
        $modx->event->output($corePath.'tv/input/');
        break;
    case 'OnTVOutputRenderList':
        $modx->event->output($corePath.'tv/output/');
        break;
    case 'OnTVInputPropertiesList':
        $modx->event->output($corePath.'tv/inputoptions/');
        break;
    case 'OnTVOutputRenderPropertiesList':
        $modx->event->output($corePath.'tv/properties/');
        break;
    case 'OnManagerPageBeforeRender':
        break;
}

It's most likely a MODX permission issue since that message comes from the processor.

You can test the by making yourself a 'sudo' user. If that fixes it, it's definitely a MODX permission issue. That means you're missing a permission in the 'mgr' context that's required by one of the processors you're calling (or one of its parent classes).

Try putting a return statement at the end of the plugin.