I also had issues with the previews. Some previews were shown and others not shown in the mobile app.
For other accounts of the instance it worked fine.
The solution was to replace the whole preview folder with a fresh and empty one.
This is the way to do it: https://help.nextcloud.com/t/how-to-clean-up-nextcloud-data-appdata-and-updater/123746/7
I recommend to set the preview size to a smaller value since the preview folder can become quite big otherwise.
Example:
‘preview_max_x’ => 1080,
‘preview_max_y’ => 1920,
Preview generation is now working really fine and fast on the fly with no more issues.
You can kind of observe the first generations when the folders 0 - f are created in the preview folder.
The size with first previews generated now (no automatic generation by job, only scrolling a bit in the app and browser):
33M preview/
Before (autmoatic preview generation by cron job was active):
21G preview_old/
One might say 21G is not too much - but it took about 15 mins to only do a du -hs on the folder. Just thinking on backup and stuff…
I therefore decided to not generate previews in a job any more.
For performance reaons I also tried to move only the preview folder to a SSD and point to it via symlink. This did’t work!
Preview generation just stopped. In the log: “30.0.4.1”,“exception”:{“Exception”:“OCP\Files\ForbiddenException”,“Message”:“Following symlinks is not allowed”.