Yes, there is a fix to this already. At the moment you would need to manually apply it by editing /usr/share/webmin/web-lib-funcs.pl file, updating the following subroutine:
sub to_ip6address
{
my ($host) = @_;
my @rv;
if (&check_ip6address($host)) {
@rv = ( $host ); # Already in v6 format
}
elsif (&check_ipaddress($host)) {
@rv = ( ); # A v4 address cannot be v6
}
elsif (!&supports_ipv6()) {
@rv = ( ); # v6 lookups not supported
}
else {
# Perform IPv6 DNS lookup
eval {
my @ai = getaddrinfo($host, undef, AF_INET6(), SOCK_STREAM);
while(@ai) {
my ($inaddr, @newai);
(undef, undef, undef, $inaddr, undef, @newai) = @ai;
if ($inaddr) {
my $addr;
(undef, $addr) = unpack_sockaddr_in6($inaddr);
push(@rv, inet_ntop(AF_INET6(), $addr));
}
@ai = @newai;
}
};
}
return wantarray ? @rv : $rv[0];
}
… and restarting Webmin afterwards with /etc/webmin/restart.