Ah I just read
DNS-Rebind on the other side will just ensure that IPv6 is working to connect to your NextBox, this is required sometimes depending on your internet-service-provider. Namely, if you get a true dual-stack (IPv4 + IPv6) connection from your ISP, then you should not even need IPv6. But if you’re on a DS-Lite connection, you might need IPv6 to access your NextBox. (see DNS Rebind Protection — Nitrokey Documentation )
from Daringer’s first post here
It doesn’t become quite as clear in the respective docs, might be worth adding Daringer’s comment with the following bottomline: Sometimes, IPv4 is NOT enough for some folks, you may need to set up IPv6! 
After I enabled IPv6 via DNS rebind whitelisting by whitelisting myname.dedyn.io, I can continue.