Hi @MikeMcQ ,
thank you for your reply. feste-ip.net is a mapping service since I only have a public ipv6 address and a natted ipv4. So requests coming from ipv4 only hosts are mapped to ipv6 by this servcie.
I supposed that they changed something because in the past, they where transparent to the acme challenge. I guess I need to ask the named service if they changed something in regards to acme challenges.