I found the problem on daffy1
curl -6v https://login2.opensuse.org
failed because its own IPv6 addr had changed, but /etc/hosts still
contained the old 2620:113:80c0:8::161 IP for login2.
The difference to wiki probably lies in these macro calls in
/etc/apache2/vhosts.d/forums.opensuse.org.conf
Use buildserver forums.opensuse.org forums
https://login2-ucs.opensuse.org/ opensuse.org
http://forum.infra.opensuse.org:80 http://forum.infra.opensuse.org:80 ...
vs
Use buildserver en.opensuse.org en https://login2-ucs.opensuse.org/
opensuse.org http://proxy-opensuse-ha.login.infra.opensuse.org:80
http://proxy-opensuse-ha.login.infra.opensuse.org:80
The last 2 params probably matter. They are named
$proxyurl $redirurl
used in
RewriteRule ^/((?!error|ICSLogin|cmd/ICSLogout|ICHAINLogout).*)
$redirurl/$1 [P,L]