Domain in Plesk can not be secured with a Let's Encrypt certificate: SERVFAIL looking up A for example.com

Symptoms

• A Let's Encrypt certificate can not be issued for a domain in Plesk with the following error:

  CONFIG_TEXT: ERR [extension/letsencrypt] Domain validation failed for example.com: Invalid response from https://acme-v02.api.letsencrypt.org/acme/authz-v3/76965942050.
  Details:
  Type: urn:ietf:params:acme:error:dns
  Status: 400
  Detail: DNS problem: SERVFAIL looking up A for example.com - the domain's nameservers may be malfunctioning; DNS problem: SERVFAIL looking up AAAA for example.com - the domain's nameservers may be malfunctioning
  ERR [extension/letsencrypt] Domain validation failed: Invalid response from https://acme-v02.api.letsencrypt.org/acme/authz-v3/76965942050.
  Details:
  Type: urn:ietf:params:acme:error:dns
  Status: 400
  Detail: DNS problem: SERVFAIL looking up A for example.com - the domain's nameservers may be malfunctioning; DNS problem: SERVFAIL looking up AAAA for example.com - the domain's nameservers may be malfunctioning

• DNS is not managed in Plesk, but externally.

• Free online checkers reports SERVFAIL error for this domain:

  https://unboundtest.com
  https://letsdebug.net

Cause

Unclear.

Resolution

Contact the external DNS side for further investigation of the issue with nameservers.
Additionally, you may consider contacting Let's Encrypt community: https://community.letsencrypt.org/.