Hmm… assuming that you can connect to other places from that system, and just not the ACMEv2 API, it might be the DoS mitigation block. Did you recently acquire this IPv6 space, or might it have been used for "bad things" relatively recently?
However, the domains you posted don't have AAAA DNS records, but you're connecting from the server to the IPv6 address for the API. Can you share the IPv6 address you're trying this from? And when you do so, probably tag @lestaff so they can take a look. It's a holiday weekend in the US, though, so I don't know what their response time will be to forum posts.