I'm using [Traefik](https://traefik.io/traefik) in my server which has built-in feature of generating Let's Encrypt certificates for my domains.

However, when I deploy my traefik stack, I get this error:

```
Unable to obtain ACME certificate for domains \"domain.com, www.domain.com\": 
unable to generate a certificate for the domains [domain.com www.domain.com]: 
error: one or more domains had a problem:\n[domain.com] acme: error: 400 
:: urn:ietf:params:acme:error:connection 
:: Timeout during read (your server may be slow or overloaded), url: \n" 
providerName=le.acme 
routerName=stag-domain-com-proxy-https@docker 
rule="Host(`domain.com`) || Host(`www.domain.com`)"
```

Some observations: 

- I have other domains in my stack and traefik was able to generate certificates for those. (e.g. it was able to create certificates for sub1.domain.com, sub2.domain.com)

- I can ping my ipv4 address, domain.com, www.domain.com and also the ipv6 address using this [tool](https://tools.keycdn.com/ipv6-ping)

- It was suggested in the [Traefik forum](https://community.containo.us/t/traefik-not-getting-ssl-certificates-for-some-domains/868/5) to check...

	> that you can curl on both IPv4 and IPv6 the ip mentioned in the A and AAAA records for the faulty 	domains?

	When I run `curl my.ipv4.addr.ess` -- I get a `404 page not found` and when I `curl  my:ip::v:6:add:ress` I get `curl: (3) IPv6 numerical address used in URL without brackets`

---

What could be causing this issue with generating certificates?