Adding external flash drive fails


I need help, because I cannot see the external flash drive I have attached to my Raspberry 3 running the NextCloudPi Docker image.
The flash drive is automatically mounted via fstab to mnt/ncstick1 and works fine. In a console I can copy and edit files there. The drive was not mounted, when I installed the docker image.
Creating a folder for it in NextCloudPi seems to work. At least I get a green icon when I click the check. I use
Folder name: ncstick1
External storage: local
No authentification
Configuration: /mnt/ncstick1
Available for: ncb, admin

If I click ‘configuration’ in the user menu and choose ‘external storage’ on the left under ‘administration’ I can see the folder. But if I click the icon, it goes to the ‘All files’ view where the ‘ncstick1’ folder does not exist.

What can I do to find out what is going wrong ?

Nextcloud version:
Operating system: Raspbian
Installed via Docker

Thanks in advance and best regards,