I thought I’d wake this thread back up after finding this via a Google search as I’ve just experienced the same issue after installing the Google-Drive External Files App.
PHP Fatal error: Interface ‘OCA\Files_External\Lib\Config\IBackendProvider’ not found in /usr/local/www/nextcloud/apps/files_external_gdrive/lib/AppInfo/Application.php on line 32
root@nextcloud:/usr/local/www/nextcloud # sudo -u www-data php /var/www/nextcloud/occ app:disable files_external_gdrive
sudo: unknown user: www-data
sudo: unable to initialize policy plugin
root@nextcloud:/usr/local/www/nextcloud # sudo -u www-data php /var/www/nextcloud/occ status
sudo: unknown user: www-data
sudo: unable to initialize policy plugin
I assume NextCloud fatally damaged. I’ll do a re-install since this is a initial setup.
I posted here…