Hi @talionarwork,
Your problem is exactly this Certbot - IPV6 Not Configured and Challenges Fail (IPV6 Preferred) as @mnordhoff explain, if you have 2 ips, one for ipv4 and the other one for ipv6, Let’s Encrypt will prefer the ipv6 and your site is not answering correctly in ipv6.
Testing ipv4 connection (OK):
curl -i4 'http://unanalytics.com/.well-known/acme-challenge/ZACZeLOKjIEC0Mr8hHHQwGrxlLdud5wHkCIu7N19YZ0'
HTTP/1.1 200 OK
Server: nginx/1.11.8
Date: Mon, 22 May 2017 11:07:24 GMT
Content-Type: text/plain; charset=utf-8
Content-Length: 87
Last-Modified: Mon, 22 May 2017 08:01:22 GMT
Connection: keep-alive
Vary: Accept-Encoding
ETag: "59229ad2-57"
Accept-Ranges: bytes
ZACZeLOKjIEC0Mr8hHHQwGrxlLdud5wHkCIu7N19YZ0.HhzheAQ6RwYUzbBNLNfso6rYoaV5GokXPgxTEzE75PA
Testing ipv6 connection (FAIL):
curl -i6 'http://unanalytics.com/.well-known/acme-challenge/ZACZeLOKjIEC0Mr8hHHQwGrxlLdud5wHkCIu7N19YZ0'
HTTP/1.1 404 Not Found
Server: nginx/1.11.8
Date: Mon, 22 May 2017 11:07:18 GMT
Content-Type: text/html
Content-Length: 169
Connection: keep-alive
Vary: Accept-Encoding
<html>
<head><title>404 Not Found</title></head>
<body bgcolor="white">
<center><h1>404 Not Found</h1></center>
<hr><center>nginx/1.11.8</center>
</body>
</html>
Good luck,
sahsanu