I have installed this preview generator app, but it doesn’t seem to do anything… and is poorly documented, if not at all… 
My cron job
*/30 * * * * php -f /var/www/nextcloud/occ preview:pre-generate
runs without error under the user www-data. (checked in my syslog)
I have remarked that in the “/var/www/nextcloud/apps/previewgenerator/lib/Command” directory, there is 3 possible command :
DeleteOld
Generate
PreGenerate
Is the differences in capital letters and writing of “pre-generate” in the cron and “PreGenerate” in the file could be the problem ?
Also, my /var/www/nextcloud/apps/previewgenerator/appinfo/database.xml seems not updated and the oc_preview_generation table in the nextcloud database empty…