Once your DNS resolves openacs.org with an IPv6 address, you have at least minimal support for IPv6 on that machine.
I've just checked access to https://openacs.org via IPv6:
ns_http run https:/openacs.org/repository/
returns as expected the full page.
ns_http run {https://[2001:628:404:74::31]/repository/}
returns the redirect to the canonical URL
status 301 time 0:10582 headers d4 body {status 301 time 0:10582 headers d4 body {<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 4.01//EN">
<html>
<head>
<title>Redirection</title>
</head>
<body>
<h2>Redirection</h2>
<a href="https://openacs.org/repository/">The requested URL has moved permanently here.</a>
<p align='right'><small><i>NaviServer/4.99.17 on https://openacs.org</i></small></p>
</body></html>
} https {sslversion TLSv1.3 cipher TLS_AES_256_GCM_SHA384}
So, the IPv6 setup on openacs.org is correct. The current access log shows 23k IPv6 requests to openacs.org, about 2% of the requests.
Concernging your two machines: do both have the exactly the same versions of the OS and NaviServer and OpenACS installed?
