Over IPv4, yeah. Not over IPv6:
$ curl -6 -i http://lexg.nl/.well-known/acme-challenge/test.txt
HTTP/1.1 404 Not Found
Server: nginx
Date: Wed, 28 Feb 2018 09:14:57 GMT
Content-Type: text/html
Content-Length: 162
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</center>
</body>
</html>
Your nginx virtualhost for that domain looks like it's only bound to IPv4. You need to tell nginx to also respond on IPv6 for that virtualhost, or withdraw the AAAA record for the domain.
Let's Encrypt will always prefer the AAAA record for a domain, if it exists.