I succed creating/running a cronjob for the preview app, but an output to some log would be much appreciated.
Seem some discussion was initiated on GitHub some time ago, but I (shame) can’t recall it.
Ass the .occ preview command used by cronjob is php, the usual &>1 kind of output is not possible.
My only way is to journalctrl around the time my cronjob is running and check for errors:
03:30:01 nextcloud CRON: (www-data) CMD (/usr/bin/nice /usr/bin/php -f /var/www/html/occ preview:pre-generate)
A more explicit like this could be really nice: