Can't issue Let's Encrypt certificate: Could not obtain directory: cURL error 6: Could not resolve host: acme-v02.api.letsencrypt.org

Symptoms

• Unable to issue Let's Encrypt certificates. The following error is displayed in Plesk:

  Error: Could not issue a Let's Encrypt SSL/TLS certificate for example.com.
  Failed to connect to the Let's Encrypt server https://acme-v02.api.letsencrypt.org.
  Please try again later or report the issue to support.
  Details
  Could not obtain directory: cURL error 6: Could not resolve host: acme-v02.api.letsencrypt.org (see http://curl.haxx.se/libcurl/c/libcurl-errors.html)

• The server cannot reach any external host via CLI. System-wide DNS resolution is failing:

  # curl -ILk https://google.com
  curl: (6) Could not resolve host: google.com

  # nslookup acme-v02.api.letsencrypt.org
  DNS request timed out.
  timeout was 2 seconds.

Cause

A network issue is preventing the server from resolving or reaching external hosts (DNS or Firewall issues).

Resolution

The issue is not related to Plesk configuration.

To resolve the problem, contact the hosting provider or a network administrator with the details of the issue, sharing the output from the examples above.