Devopol

https://apps.nextcloud.com/apps/previewgenerator https://docs.nextcloud.com/server/latest/admin_manual/configuration_files/previews_configuration.html

The first time you install this app, before using a cron job, you properly want to generate all previews via: ./occ preview:generate-all -vvv

To enable pre-generation of previews you must add php /var/www/nextcloud/occ preview:pre-generate to a system cron job that runs at times of your choosing. → each day

Debugging:

If CLI (occ) isn’t working go to config.php and set maintenance⇒true then occ app:list; occ app:disable [app name]