Jellyfin app on TrueNas Scale "There was an error adding the media path"

[Noctran]

When attempting to set up Jellyfin as an app on my TrueNAS Scale server, adding a media library using the wizard throws the error "There was an error adding the media path. Please ensure the path is valid and Jellyfin has access to that location" as a popup in the lower left corner.

   

This happens both with the existing app as well as with a custom app Docker Compose YAML.
The error happens with a custom user, the integrated apps user and also with root used.
For troubleshooting I have created a dedicated dataset in the root of my HDD pool "/mnt/tank/testing-jelly" owned by the apps user and group (with additional full permission flags for the apps user and group). Under that I created /config /cache /movies. When launching the container, /config and /cache get populated so they should be used as planned. When selecting the correct Dir for the Library in the Wizard it displays existing Folders under /movies if there are any, so Jelly actually has access.
There is no log entry created when the error gets displayed.
I'm aware that this may be an issue with TrueNAS, but I don't know how I could determine that or even narrow the potential error source down.

I initially asked for help in the Matrix Chat. After we ran it through I was sent to create a Github Issue but that got closed because its not a Jellyfin bug. I'm unsure where else to seek help, tho😅

Reproduction steps

1. Create the TrueNAS Scale app, either existing or custom.
   
2. Access the web interface
   
3. Via the Getting Started Wizard or from the Dashboard, use the "Add Media Library."
   
4. Attempt to add the library and press the OK button
   

Environment
- OS:TrueNas Scale 25.04.1
- Virtualization: Docker
- Clients: Browser
- Browser: current Firefox & Chrome
- Networking: Bridge
- Jellyfin Data Storage: local ZFS Pool using Sata SSDs or HDDs
- Media Storage: local ZFS Pool using Sata HDDs

Additional information
The Docker Compose instructions used (indentation is correct when applied):
services:
jellyfin:
container_name: jellyfin
environment:
- TZ=Europe/Berlin
image: ghcr.io/jellyfin/jellyfin:latest
network_mode: bridge
ports:
- 8096:8096/tcp
restart: unless-stopped
user: '0:0'
volumes:
- /mnt/tank/testing-jelly/config:/config:rw
- /mnt/tank/testing-jelly/cache:/cache:rw
- /mnt/tank/testing-jelly/media/movies:/movies:rw