Could it be that it is IPv6 specific?
wget -vS http://cert.int-x3.letsencrypt.org/
--2018-03-09 19:10:45-- http://cert.int-x3.letsencrypt.org/
Resolving cert.int-x3.letsencrypt.org (cert.int-x3.letsencrypt.org)... 2a02:26f0:132:3a6::ce0, 2a02:26f0:132:3bc::ce0, 104.127.50.7
Connecting to cert.int-x3.letsencrypt.org (cert.int-x3.letsencrypt.org)|2a02:26f0:132:3a6::ce0|:80... connected.
HTTP request sent, awaiting response...
HTTP/1.1 301 Moved Permanently
Server: AkamaiGHost
Content-Length: 0
Location: https://cert.int-x3.letsencrypt.org/
Cache-Control: max-age=0
Expires: Fri, 09 Mar 2018 19:10:45 GMT
Date: Fri, 09 Mar 2018 19:10:45 GMT
Connection: keep-alive
Location: https://cert.int-x3.letsencrypt.org/ [following]
--2018-03-09 19:10:45-- https://cert.int-x3.letsencrypt.org/
Connecting to cert.int-x3.letsencrypt.org (cert.int-x3.letsencrypt.org)|2a02:26f0:132:3a6::ce0|:443... connected.
The certificate's owner does not match hostname ‘cert.int-x3.letsencrypt.org’