I did some further testing and setup a Vanilla server following the official Wiki trying to see if i had the same result in another distro.
Doing so I found the problem.
When setting up ddclient it defaults to IPv4. That means, the dynDNS entry had an A Record for an IPv4 Adress (one from a private network).
After the changes to the ddclient.conf the IPV4 Adress was never updated, but also was not removed.
The AAAA Record (IPv6) was just added.
Looks like some Software (i.e. browser I used to acces Webui) was smart enough to try the IPv6 Adress from the AAAA Record if they cannot reach the A Record IPv4.
Other Software (i.e. Minecraft Launcher) defaults just doesnt bother looking at AAAA Records if there is an A Record. no matter if reachable or not.
I manually reset the dynDNS information with my provider and ddclient -force an update.
=> no A record, AAAA Record = IPv6
It’s working now.
Sorry for wasting your time. (and mine XD)