Problem obtaining ACME certificate for domain

Traefik Traefik v2
,
1

I have a problem with requesting a SSL certificate for my domain: example.com.

Normally I only request SSL certificates for subdomains: subdomain.example.com and that works great.

What I wish to accomplish is that my website is reachable from https://example.com and https://www.example.com.

For that, I added two A Records in my DNS management:
@ and www, both pointing to the correct IP.

However when I start up my container, Traefik is only able to create a certificate for www.example.com, but not example.com

I get the following error:

time="2022-01-14T09:16:35+01:00" level=error msg="Unable to obtain ACME certificate for domains "www.example.com,example.com": unable to generate a certificate for the domains [example.com]: error: one or more domains had a problem:\n[example.com] acme: error: 403 :: urn:ietf:params:acme:error:unauthorized :: Invalid response from http://example.com/.well-known/acme-challenge/YTzjib2ZgthmXH9pOuDmc2PiD62O78hy5jH_xWjKm34 [IPv6 address]: "\n\n404 Not Found\n\n

Not Found

\n<p"\n" providerName=letsencrypt.acme routerName=app@docker rule="Host(www.example.com,example.com)"

docker-compose.yml

version: '3.7'

services:
  web:
    build:
      context: ./
      dockerfile: Dockerfile
    command: gunicorn app.wsgi:application --bind 0.0.0.0:8001 --timeout 0
    expose:
      - 8001
    networks:
      - proxy
    labels:
      - "traefik.enable=true"
      - "traefik.docker.network=app"
      - "traefik.http.routers.app.entrypoints=websecure"
      - "traefik.http.routers.app.rule=Host(`www.example.com`,`example.com`)"
      - "traefik.http.services.app.loadbalancer.server.port=8001"

networks:
  proxy:
    external: true

I thought the problem was the [IPv6 address], so I deleted all the AAAA Records. But that doesn't explain why the www.example.com worked, right..? I did a check at https://ipv6-test.com/validate.php and they couldn't find any AAAA records.

Why can I only create a certificate for www.example.com, but not example.com? Am I missing DNS records?