Here's an interesting one.
I've installed Magic Preview and when I click the button on any resource, I get the following error;
PHP Fatal error: Cannot declare class modResourceUpdateProcessor, because the name is already in use in /var/www/core/model/modx/processors/resource/update.class.php on line 0
PHP message: PHP Stack trace:PHP message: PHP
1. {main}() /var/www/assets/components/magicpreview/connector.php:0
2. modConnectorRequest->handleRequest($options = ['processors_path' => '/var/www/core/components/magicpreview/processors/', 'location' => ''])
3. modConnectorRequest->prepareResponse($options = ['processors_path' => '/var/www/core/components/magicpreview/processors/', 'location' => '', 'action' => 'resource/preview-v2'])
4. modConnectorResponse->outputContent($options = ['processors_path' => '/var/www/core/components/magicpreview/processors/', 'location' => '', 'action' => 'resource/preview-v2'])
...
The preview window opens and stays on "Preparing your preview", the resource has the "Please Wait..." overlay and its console shows the HTTP 500 error.