The export configuration functionality provided at admin/config/development/export doesn't work when Drupal is installed in a subdirectory of document root (e.g. mysite.com/drupal/).
Configuration export button form action path is now set to absolute path starting with '/':
$form['#action'] = '/admin/config/development/export-download';
File: core/modules/config/lib/Drupal/config/Form/ConfigExportForm.php
The form action path should start with base_path().