Portainer behind Traefik - Bad certificate - insecureSkipVerify: true does not kick in?

Traefik Traefik v2
2

From the IP I would say the TLS error happens between browser client and Traefik, insecureSkipVerify is used between Traefik and target service.

I don't think there are any complications with Portainer in general, here an example compose file for stack deploy with Docker Swarm:

version: '3.9'

services:
  agent:
    image: portainer/agent:2.18.2
    environment:
      - LOG_LEVEL=DEBUG
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
      - /var/lib/docker/volumes:/var/lib/docker/volumes
    networks:
      - agent_network
    deploy:
      mode: global
      placement:
        constraints: [node.platform.os == linux]

  portainer:
    image: portainer/portainer-ce:2.18.2
    command: -H tcp://tasks.agent:9001 --tlsskipverify
    #ports:
    #  - "9443:9443"
    #  - "9000:9000"
    #  - "8000:8000"
    volumes:
      - portainer_data:/data
    networks:
      - agent_network
      - proxy
    deploy:
      mode: replicated
      replicas: 1
      placement:
        constraints:
          - node.role == manager
          - node.hostname == server1
      labels:
        - "traefik.enable=true"
        - "traefik.docker.network=proxy"
        - "traefik.http.routers.portainer.entrypoints=websecure"
        - "traefik.http.routers.portainer.rule=Host(`portainer.example.com`)"
        - "traefik.http.services.portainer.loadbalancer.server.port=9000"
        - "traefik.http.services.portainer.loadbalancer.passhostheader=true"

networks:
  agent_network:
    driver: overlay
    driver_opts:
      com.docker.network.driver.mtu: 1400
    attachable: true
  proxy:
    external: true

volumes:
  portainer_data: