FireFly III with csv-importer behind taefik 1.7

Hello,
I'm trying to run FireFly III (a finance manager https://www.firefly-iii.org/) additionally on my VPS where I have traefik 1.7.26 sucessfully running with multiple services. The main app, jc5x/firefly-iii, is running flawlessly. But I have problems with the additional fireflyiii/csv-importer. I have to admit that I'm not really into the topic of reverse-proxies and gathered once a quite good configuration of traefik labels to get nearly everything running, so I'm a bit lost now and don't know with which configuration and/or debugging steps I could eliminate the remaining problems.

The problem is that the webpage of the csv-importer does not load correctly if ran behind traefik. If I run it on a random port besides traefik everything loads perfectly so I assume the problem is related to traefik.
Also there is a hint in the docs of the csv-importer which says to make sure that multiple headers are redirected properly (X-Forwarded-Host, X-Forwarded-Server, X-Forwarded-Proto, X-Forwarded-For, Host) and I think my problem could be related to this. https://firefly-iii.gitbook.io/firefly-iii-csv-importer/installing-and-running/docker#docker-and-reverse-proxies
But I really have no clue how to add these options to traefik. I searched around a lot and played arount with the labels:

     - "traefik.frontend.auth.forward.trustForwardHeader=true"
     - "traefik.frontend.whiteList.SourceRange=172.18.0.0/16"
     - "traefik.frontend.whiteList.useXForwardedFor=true"
     - "traefik.frontend.headers.SSLProxyHeaders=X-Forwarded-Proto:https"
     - "traefik.frontend.headers.SSLProxyHeaders=X-Forwarded-Host:172.18.0.15"
     - "traefik.frontend.headers.SSLProxyHeaders=X-Forwarded-Server:172.18.0.15"
     - "traefik.frontend.passHostHeader=true"

but they didn't help or I did not the right adjustments to them...

Maybe someone has a hint or can give me some advise how to debug it.

Here is my docker-compose.yml

version: '3.3'

services:
fireflyiii:
image: jc5x/firefly-iii:latest
volumes:
- firefly_iii_export:/var/www/firefly-iii/storage/export
- firefly_iii_upload:/var/www/firefly-iii/storage/upload
env_file: .env
depends_on:
- fireflyiiidb
restart: unless-stopped
labels:
- "traefik.enable=true"
- "traefik.backend=firefly"
- "traefik.frontend.rule=Host:firefly.mydomain"
- "traefik.port=8080"
- "traefik.docker.network=traefik_proxy"
- "traefik.frontend.headers.SSLRedirect=true"
- "traefik.frontend.headers.STSSeconds=315360000"
- "traefik.frontend.headers.browserXSSFilter=true"
- "traefik.frontend.headers.contentTypeNosniff=true"
- "traefik.frontend.headers.forceSTSHeader=true"
- "traefik.frontend.headers.SSLHost=mydomain"
- "traefik.frontend.headers.STSIncludeSubdomains=true"
- "traefik.frontend.headers.STSPreload=true"
- "traefik.frontend.headers.frameDeny=true"
networks:
- traefik_proxy
- firefly

fireflyiiidb:
image: yobasystems/alpine-mariadb:latest
environment:
- MYSQL_RANDOM_ROOT_PASSWORD=yes
- MYSQL_USER=firefly
- MYSQL_PASSWORD=DBPWD
- MYSQL_DATABASE=firefly
volumes:
- firefly_iii_db:/var/lib/mysql
restart: unless-stopped
networks:
- firefly

cron:
image: alpine
command: sh -c "echo "0 3 * * * wget https://firefly.mydomain/cron/run/TOKEN" | crontab - && crond -f -L /dev/stdout"
restart: unless-stopped

fireflyiiicsv:
image: fireflyiii/csv-importer:latest
environment:
- FIREFLY_III_ACCESS_TOKEN=xyz
- FIREFLY_III_URI=https://firefly.mydomain
labels:
- "traefik.enable=true"
- "traefik.backend=fireflycsv"
- "traefik.frontend.rule=Host:fireflycsv.mydomain"
- "traefik.port=80"
- "traefik.docker.network=traefik_proxy"
- "traefik.frontend.headers.SSLRedirect=true"
- "traefik.frontend.headers.STSSeconds=315360000"
- "traefik.frontend.headers.browserXSSFilter=true"
- "traefik.frontend.headers.contentTypeNosniff=true"
- "traefik.frontend.headers.forceSTSHeader=true"
- "traefik.frontend.headers.SSLHost=mydomain"
- "traefik.frontend.headers.STSIncludeSubdomains=true"
- "traefik.frontend.headers.STSPreload=true"
- "traefik.frontend.headers.frameDeny=true"
networks:
- traefik_proxy

volumes:
firefly_iii_export:
firefly_iii_upload:
firefly_iii_db:

networks:
traefik_proxy:
external:
name: traefik_proxy
firefly:
external: false