The parent servers are not providing glue for all your nameservers. This means that they are supplying the NS records (host.example.com), but not supplying the A records.
That may cause some extra milliseconds in DNS. This will usually occur if your DNS servers are not in the same TLD as your domain
Some servers does not provide A records for nameservers.
missing A record(s) for (ns1.first-ns.de,robotns2.second-ns.de,robotns3.second-ns.com) at nameserver 188.8.131.52
missing A record(s) for (ns1.first-ns.de,robotns2.second-ns.de,robotns3.second-ns.com) at nameserver 184.108.40.206
missing A record(s) for (ns1.first-ns.de,robotns2.second-ns.de,robotns3.second-ns.com) at nameserver 220.127.116.11
Nameservers report identical NS records
The NS records at all your nameservers are identical.
All of your nameservers listed at the parent nameservers responded.
Nameserver name validity
All of the NS records that your nameservers report are valid (no IPs or partial domain names).
Number of nameservers
You have 3 nameservers. You must have at least 2 nameservers (RFC2182 section 5 recommends at least 3 nameservers), and preferably no more than 7.
All the nameservers listed at the parent servers answer authoritatively for your domain.
Missing (stealth) nameservers
All your nameservers are also listed at the parent servers.
Root missing nameservers
All of the nameservers listed at the parent nameservers are also listed as NS records at your nameservers.
Nameservers on separate class C's
Nameservers are in a different networks.
All of your NS records appear to use public IPs.
SOA record is:
All your nameservers agree that your SOA serial number is 2018041900 That means that all your nameservers are using the same data.
SOA (Start of Authority) record states that your master (primary) name server is: ns1.first-ns.de That server is listed at the parent servers, which is correct.
SOA serial number is: 2018041900 This appears to be in the recommended format of YYYYMMDDnn, where 'nn' is the revision. This number must be incremented every time you make a DNS change.
SOA Retry interval is : 14400 seconds. This seems OK. (Values about 3600-7200 seconds is good if not using DNS NOTIFY; RFC1912 2.2 recommends a value between 1200 to 43200 seconds (20 minutes to 12 hours)). This value determines how often secondary/slave nameservers check with the master for updates.
SOA Retry interval is : 1800 seconds. This seems OK. (Values about 120-7200 seconds is good). The retry value is the amount of time your secondary/slave nameservers will wait to contact the master nameserver again if the last attempt failed.
SOA Expire time is : 604800 seconds. This seems OK. (Values 604800 to 2419200 seconds (1-4 weeks) is good). RFC1912 suggests 2-4 weeks. This is how long a secondary/slave nameserver will wait before considering its DNS data stale if it can't reach the primary nameserver.
SOA Expire time is : 86400 seconds. This seems OK. (about 300 to 86400 seconds or 5 min - 24 hours is good). RFC2308 suggests a value of 1-3 hours. This value used to determine the default (technically, minimum) TTL (time-to-live) for DNS entries, but now is used for negative caching.
Trace to visitor-analytics.io
lookup visitor-analytics.io at A.ROOT-SERVERS.NET(18.104.22.168) 21 ms
A.ROOT-SERVERS.NET(22.214.171.124) refer to a0.nic.io(126.96.36.199)
lookup visitor-analytics.io at a0.nic.io(188.8.131.52) 85 ms
a0.nic.io(184.108.40.206) refer to robotns3.second-ns.com(unknow IP)
extra IP lookup for robotns3.second-ns.com at root server A.ROOT-SERVERS.NET(220.127.116.11)
lookup robotns3.second-ns.com at A.ROOT-SERVERS.NET(18.104.22.168) 38 ms
A.ROOT-SERVERS.NET(22.214.171.124) refer to a.gtld-servers.net(126.96.36.199)
lookup robotns3.second-ns.com at a.gtld-servers.net(188.8.131.52) 18 ms
a.gtld-servers.net(184.108.40.206) refer to ns.second-ns.com(220.127.116.11)
lookup robotns3.second-ns.com at ns.second-ns.com(18.104.22.168) 142 ms
got A record 'robotns3.second-ns.com IN A 22.214.171.124' from robotns3.second-ns.com(126.96.36.199)
lookup visitor-analytics.io at robotns3.second-ns.com(188.8.131.52) 82 ms
got A record 'visitor-analytics.io IN A 184.108.40.206' from visitor-analytics.io(220.127.116.11)