Frequent error codes returned by NAST nameserver checker:
1.) error 903 - port 53 is not opened, check it using the port scanner.
2.) error 901 - nameserver refuses to answer. Maybe the domain is not in the zone of this nameservers.
3.) error 110 - "retry" value for SOA is not match required. Check current with the command: SOA @your_nameserver.de your_domain.de +multiline than check to correct.
Allowed parameters in seconds are 450 - 1200.
4.) error 118 - message means that the nameserver entries (NS records) in the DNS entry for the corresponding domain do not match the nameservers specified for the domain. Either the zone file or the entries need to be adjusted.