dns-01 let's encrypt seems to be broken

On debian 8 with:

Webmin version 1.910
Usermin version 1.760
Virtualmin version 6.06-2 Pro
Cloudmin version 9.4 Pro

it seem that the "dns validation" ist broken. testet in:

Webmin -> SSL Encryption -> Let's Encrypt:

Other directory: /var/www/html  =  validation OK
Use DNS domains for validation  = gave up

if I issue a certificate for a domain that got a vhost in apache but no domain in virtualmin

Virtualmin -> {domain} ->  Server Configuration -> SSL Certificate -> Let's Encrypt:

request failed : Web-based validation failed : Failed to request certificate
{domain} challenge did not pass: Invalid response from http://{domain}/.well-known/acme-challenge/......xsaAZ0g [{IPv6}]: "<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">\n<html><head>\n<title>404 Not Found</title>\n</head><body>\n<h1>Not Found</h1>\n<p"

That is ok because there is no "/.well-known/acme-challenge" and virtualmin could not create it there. but the next is not ok

DNS-based validation failed : Failed to request certificate :
Gave up waiting for validation

I've testet it with and WITHOUT ipv6 on that server