DENIC, which is the .de registry, has set out a number of rules for nameservers. The following two requirements play a role in almost all .de domain errors you encounter.
Firstly, all nameservers are not permitted on the same subnet. For instance, this means that 18.104.22.168 and 22.214.171.124 are not allowed because the first three numbers are the same. Change your nameservers and use an additional nameserver located in a different subnet. You may also use our secondary DNS service free of charge.
Secondly, certain rules apply to the SOA record content on your domain. The error message will already indicate what should be improved.