So... #fritzbox has #dns #rebind protection.

Which wasn't a problem when I hosted a server with a natted ipv4, but is a problem with a non-natted #ipv6.

You can set exceptions per #domainname (not per ipv6 or per host 😠).
Docs state that you need to specify every subdomain (😰), but listing the main domain seems to suffice.

In the exceptions of #rebind protection of a #fritzbox, you can use wildcards.