If enabled, any error on the server side will be sent in the response. Set the debug option to true in uploader/index.php.
$config['debug'] = true;If you get an unexpected error (Oops! Something went wrong.), you can use browser dev tools to see the errors and response from the server.
In Google Chrome, right-click and select Inspect, or use the keyboard shortcut: Ctrl+Shift+I (or Cmd+Opt+I on Mac).
Navigate to the Console tab to see potential JavaScript errors and the Network tab to see the requests/responses when you upload files (make sure Record Network Log is on). Watch this video for more.