HTTPS and subdomain in Traefik - docker

Question

I have a Traefik and a few services in docker-containers. Here is docker-compose.yml file:
version: "3"
services:
main-app:
image: some-image
container_name: main_app
labels:
- "traefik.enable=true"
- "traefik.http.routers.app.rule=Host(`domain.com`)"
- "traefik.http.routers.app.entrypoints=https"
- "traefik.http.routers.app.tls.certresolver=cert
moodle:
image: some-moodle-image
container_name: moodle
labels:
- "traefik.enable=true"
- "traefik.http.routers.moodle.rule=Host(`moodle.domain.com`)"
- "traefik.http.routers.moodle.entrypoints=https"
- "traefik.http.routers.moodle.tls.certresolver=cert"
traefik:
image: "traefik"
container_name: traefik
command:
- "--api.insecure=true"
#Entrypoints settings
- "--entrypoints.http.address=:80"
- "--entrypoints.https.address=:443"
# Providers settings
- "--providers.docker=true"
- "--providers.docker.exposedbydefault=false"
# Acme challeges settings
- "--certificatesresolvers.cert.acme.httpchallenge=true"
- "--certificatesresolvers.cert.acme.httpchallenge.entrypoint=web"
- "--certificatesresolvers.cert.acme.email=some-mail#help.me"
- "--certificatesresolvers.cert.acme.storage=/letsencrypt/acme.json"
labels:
#Redirect HTTP -> HTTPS
- "traefik.enable=true"
- "traefik.http.routers.https-redirect.entrypoints=http"
- "traefik.http.routers.https-redirect.rule=HostRegexp(`{any:.*}`)"
- "traefik.http.routers.https-redirect.middlewares=https-redirect"
- "traefik.http.middlewares.https-redirect.redirectscheme.scheme=https"
ports:
- "80:80"
- "443:443"
- "8080:8080"
volumes:
- "/letsencrypt:/letsencrypt"
- "/var/run/docker.sock:/var/run/docker.sock:ro"
As you can see the main app container is tied to the main domain (domain.com). And moodle container to the subdomain (moodle.domain.com).
Now to the heart of the matter. On the main domain, the certificate is issued and works HTTPS. But on the subdomain, I get the next error:
time="2020-05-01T13:12:54Z" level=error msg="Unable to obtain ACME certificate for domains \"moodle.domain.com\":
unable to generate a certificate for the domains [moodle.domain.com]:
error: one or more domains had a problem:[moodle.domain.com] acme: error: 403 ::
urn:ietf:params:acme:error:unauthorized ::
Invalid response from http://moodle.domain.com/.well-known/acme-challenge/(some private code)[(some ip address)]:
404, url: \n" providerName=cert.acme routerName=moodle#docker rule="Host(`moodle.domain.com`)"
How can this be caused and how can this problem be solved?