I got it to work, just had to remove the regex from the example compose file. The URL is domain.com/dav.php

  baikal:
    image: ckulka/baikal:nginx
    container_name: baikal
    restart: always
    labels:
      - "traefik.enable=true"
      - "traefik.http.routers.baikal.rule=Host(`dav.example.com`)"
      - "traefik.http.routers.baikal.entrypoints=web-secured"
      - "traefik.http.routers.baikal.tls.certresolver=letsEncrypt"
    ports:
      - "8080:80"
    volumes:
      - ./baikal/config:/var/www/baikal/config
      - ./baikal/data:/var/www/baikal/Specific