Frequent error codes returned by NAST nameserver checker:

Error 901 - Nameserver refuses to answer
Solution: check that the zone actually exists on the nameserver that was assigned to a domain
 

Error 902 - Domain update failed. Timeout (target)
Solution: Check that the nameservers are online and can respond to UDP based DNS requests. TCP based DNS requests are supported too, but should be used as a fallback approach towards prior failed UDP requests only. 

Error 903 -Timeout with recursive resolver
Solution: Internal Timeout – Please try again

Error 904 - Port is unreachable
Solution: Target not listening at UDP Port 53, make sure port is setup and opened

Error 999 - Unexpected exception
This code means that the DNS query failed.
Solution : Please contact Business Services. (NOTE: NS was not accessible over UDP/TCP on port 53)

Error 107 - Insufficient diversity of nameserver' s IPaddresses
Solution: One nameserver should differ completely from another by using different IPv4 and IPv6 addresses. Configure your nameservers with different IP addresses. 

Error 110 - SOA ‘retry’ value is less than the recommended one
Solution: increase the retry value of the SOA record (4)

Error 118 - nameserver entries (NS records) in the DNS entry for the corresponding domain do not match the nameservers specified for the domain
Solution: Review the NS (nameserver) records specified in the zone file and make sure it matches with the nameservers entered for the domain. 

For more information about .de Nameserver Predelegation check and errors, please refer to this document.