How to fix DNS_PROBE_FINISHED_NXDOMAIN?

Clients sometimes complain that periodically are unable to access the site. An error occurs, as in the screenshot:
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The problem is not constantly. The website for more than a month, i.e. all DNS Casy had already updated. The problem is most likely in the settings of the DNS records on the hosting or Registrar, but where? Where to dig?

PS: screenshots of the DNS settings can provide.
March 19th 20 at 08:54
1 answer
March 19th 20 at 08:56
First: the Problem is not one user, so the problem is not with their machines and web browsers.
Second: a problem with the DNS server responsible for the domain zone.
To check who portacot, can be a free account to register DNS and see how it will work. For example: https://www.cloudns.net Use more than 5 years service.

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