Hi @NeikiDev
It would appear linkskurz.de has DNSSEC enabled. This is currently not supported on Netlify (see feature request.) If you wish to use this domain with Netlify you will have to configure external DNS or disable DNSSEC if you wish to use Netlify DNS.
linkskurz.com shows as existing in Netlify DNS
% whois linkskurz.com | grep -i '^name server'
Name Server: DNS1.P02.NSONE.NET
Name Server: DNS2.P02.NSONE.NET
Name Server: DNS3.P02.NSONE.NET
Name Server: DNS4.P02.NSONE.NET
Although dig linkskurz.com NS returns nothing. This is likely because NS propagation is complete (or is broken) as seen using https://dnschecker.org/#NS/linkskurz.com. The domain is unreachable for me although using Opera VPN set to Americas I was able to connect to it and am redirected to neiki.dev.