Traefik Bad Gateway

I've got some strange issue. I have following setup: one docker-host running traefik as LB serving multiple sites. sites are most php/apache. HTTPS is managed by traefik. Each site is started using a docker-compose YAML containing the following:

version: '2.3'
services:
  redis:
    image: redis:alpine
    container_name: ${PROJECT}-redis
    networks:
      - internal
  php:
    image: registry.gitlab.com/OUR_NAMESPACE/docker/php:${PHP_IMAGE_TAG}
    environment:
      - APACHE_DOCUMENT_ROOT=${APACHE_DOCUMENT_ROOT}
    container_name: ${PROJECT}-php-fpm
    volumes:
       - ${PROJECT_PATH}:/var/www/html:cached
       - .docker/php/php-ini-overrides.ini:/usr/local/etc/php/conf.d/99-overrides.ini
    ports:
      - 80
    networks:
      - proxy
      - internal
    labels:
      - traefik.enable=true
      - traefik.port=80
      - traefik.frontend.headers.SSLRedirect=false
      - traefik.frontend.rule=Host:${PROJECT}
      - "traefik.docker.network=proxy"

networks:
  proxy:
    external:
      name: proxy
  internal:

(as PHP we use 5.6.33-apache-jessie or 7.1.12-apache f.e.)

Additionally to above, some sites get following labels:

traefik.docker.network=proxy
traefik.enable=true
traefik.frontend.headers.SSLRedirect=true
traefik.frontend.rule=Host:example.com, www.example.com
traefik.port=80
traefik.protocol=http

what we get is that some requests end in 502 Bad Gateway traefik debug output shows:

time="2018-03-21T12:20:21Z" level=debug msg="vulcand/oxy/forward/http: Round trip: http://172.18.0.8:80, code: 502, Length: 11, duration: 2.516057159s"

can someone help with that? it's completely random when it happens our traefik.toml:

debug = true
checkNewVersion = true
logLevel = "DEBUG"

defaultEntryPoints = ["https", "http"]
[accessLog]

[web]
address = ":8080"

[web.auth.digest]
users = ["admin:traefik:some-encoded-pass"]

[entryPoints]
  [entryPoints.http]
  address = ":80"
#    [entryPoints.http.redirect] # had to disable this because HTTPS must be enable manually (not my decission)
#      entryPoint = "https"
  [entryPoints.https]
  address = ":443"
    [entryPoints.https.tls]


[retry]

[docker]
endpoint = "unix:///var/run/docker.sock"
domain = "example.com"
watch = true
exposedbydefault = false


[acme]
email = "info@example.com"
storage = "acme.json"
entryPoint = "https"
onHostRule = true

[acme.httpChallenge]
entryPoint = "http"

Could the issue be related to using the same docker-compose.yml?

For anyone getting the same issue:

After recreating the network (proxy) and restarting every site/container it seems to work now. I still don't know where the issue was from.