DeNic, the .DE registry, has many rules and regulations surrounding nameserver changes for their domain names. As such, they have certain checks and balances in place.
How to change nameservers for a .DE domain
The first thing you need to do is run a check on the domain name and the nameservers that you would like associated with that domain name by visiting denic.de.If the nameservers fail the check, DeNic will not allow us to add the servers for you and we have no way to correct this on our side. If the check returns errors, please submit these errors to your web host and ask them to troubleshoot the issue. Also be sure that all SOA records are in place.
If/when the check yields a successful result you can add the nameservers, as you normally would for a domain. A guide on how to update the nameservers can be found at Changing nameservers for DNS management.
Note: For custom/private nameservers, please contact support and we would be happy to assist you with adding those.