No photos are displayed in the photos app

I’ve installed nextcloud on a raspberry pi 4 and all is running well, I just have one problem.
When I open the photos app on the menu no photos are displayed.
I only see the attempt to load the images that come by default in nextcloud.

I’ve tried to use

sudo -u www-data php occ preview:generate-all /media/Disk1/Preview -vvv

but I receive this:

I have removed the Preview Generator App and don’t use it anymore, but your call seems to be wrong. It should only work if that app ist installed. But that app is calling the server component only, so it looks like that ImageMagick or so is missing.