Gekko

Yeah, there are many implementations of nextcloud, i know - thanks for your help.
 
After trying to let nextcloud recreate the previews folder by using
docker exec -it nextcloud occ preview:generate-all -vvv i stepped into an error:
[OCP\Files\NotPermittedException] Could not create folder "/appdata_[MyInstanceID]/preview/4/1/0/0/9/0/5/82634"  
So i moved the content of my photos from a synced remote directory on a windows machine out of nextcloud's reach and wiped the whole preview directory with
rm -r /mnt/user/nextcloud/app_[MyInstanceID]/preview logged into mariadb via
docker exec -it mariadb mysql -h [MyMariaDBDockerContainerIP] -P 3306 --protocol=tcp -u root -p nextcloud provided the root password for mariaDB and inserted the following line:
DELETE FROM oc_filecache WHERE path LIKE "appdata\_[MyInstanceID]/preview/%";  
After that, i moved the photos folder back into the synced directory, let Nextcloud do it's magic and after having synced the content, i dropped
docker exec -it nextcloud occ files:scan --all docker exec -it nextcloud occ preview:generate-all -vvv to rebuild the preview pictures.
 
Now, the photos section not only works again, but is incredibly fast as well - it never performed so well during the whole installation.
 
I wrote this down here in unraid forums to include it the way other users can reproduce this solution if they step into the weird behavior or experience the error mentioned above. I'm hosting this installation for me and my girlfriend, so i had not to worry about a huge user population.
As far as i'm concerned, nothing was specifically unraid related regarding the origins of the weird behavior. Every command displayed here shows the way how the solution can be handled using unraid's terminal and docker logic.
 
Good luck,
Gekko
 
[Edit: spelling]