NC 11.0.1 on Ubuntu 16.04.1
The occ command preview:generate-all truncates the filenames in nextcloud.log, therefore debugging is impossible at the moment.
Example
sudo -u www-data php occ preview:generate-all
tail -f nextcloud.log
MP3->getThumbnail('\/files\/Archiv\/K...', 2048, 2048, false, O
Detailed info
https://help.nextcloud.com/t/preview-generate-all-not-working/8190/4