Bug: zurg isn't working when firewall is on and port is opened

[sfkmk]

Expected Behavior
When I activate my firewall and open Port 9999 in my firewall it should let zurg access real-debrid.

Actual Behavior
As soon as I activate my firewall with port 9999 opened, zurg is no longer able to access real-debrid data. If I open all ports or deactivate my firewall, everything's fine.

Operating System
ubuntu 22.04

Environment Setup
my docker-compose:
services:
zurg:
image: ghcr.io/debridmediamanager/zurg-testing:latest
container_name: zurg
restart: unless-stopped
ports:
- 9999:9999
volumes:
- ./zurg/plex_update.sh:/app/plex_update.sh
- ./zurg/config.yml:/app/config.yml
- zurgdata:/app/data

rclone:
image: rclone/rclone:latest
container_name: rclone
restart: unless-stopped
environment:
TZ: Europe/Berlin
PUID: 1000
PGID: 1000
volumes:
- /mnt/zurg:/data:rshared
- ./zurg/rclone.conf:/config/rclone/rclone.conf
cap_add:
- SYS_ADMIN
security_opt:
- apparmor:unconfined
devices:
- /dev/fuse:/dev/fuse:rwm
depends_on:
- zurg
command: "mount zurg: /data --allow-other --allow-non-empty --dir-cache-time 10s --vfs-cache-mode full"

volumes:
zurgdata:

Logs
Nothing related to this issue in the zurg logs, already checked.

Rclone Configuration
[zurg]
type = webdav
url = http://zurg:9999/dav
vendor = other
pacer_min_sleep = 0

[zurghttp]
type = http
url = http://zurg:9999/http
no_head = false
no_slash = false

Zurg Configuration
zurg: v1
token: CENSORED
check_for_changes_every_secs: 10
enable_repair: true
auto_delete_rar_torrents: true
on_library_update: sh plex_update.sh "$@"

directories:
anime:
group_order: 10
group: media
filters:
- regex: /\b[a-fA-F0-9]{8}\b/
- any_file_inside_regex: /\b[a-fA-F0-9]{8}\b/

shows:
group_order: 20
group: media
filters:
- has_episodes: true
- not_contains:

movies:
group_order: 30
group: media
only_show_the_biggest_file: true
filters:
- regex: /.*/