Nameserver error "Nameserver ns-b.xxxxxxxxx.zzz cannot be queried for SOA" with mirrored DNS servers

I have a multiserver setup with 2 nameservers, ns-a.xxxxxxxxx.zzz and ns-b.xxxxxxxxx.zzz . Server ns-b.xxxxxxxxx.zzz is set to be a mirror of ns-a.xxxxxxxxx.zzz . The servers are in sync (I checked the databases). Both have a firewall entry allowing UDP on port 53. Both have a set of IPv4 and IPv6 addresses in the list of server IP addresses. The servers have IP addresses in completely different IP ranges.
I have made a DNS zone in ISPConfig for a domain. The DNS zone values for that domain are replicated to nameserver ns-b.xxxxxxxxx.zzz correctly.
Yet, when I try to assign both nameservers as nameservers for this domain wth the registrar, I get an error
"Nameserver ns-b.xxxxxxxxx.zzz cannot be queried for SOA"
for nameserver ns-b.xxxxxxxxx.zzz . I used several onine tools and indeed the second nameserver ns-b.xxxxxxxxx.zzz does not return any values for the domain at all.
What could be wrong and how to set things correctly so I can start using the DNS servers ?
Thanks !
Erik

 

Here's the result:

# dig @localhost ns-b.yourdomain.tld

; <<>> DiG 9.16.1-Ubuntu <<>> @localhost ns-b.yourdomain.tld
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 56050
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
; COOKIE: 22a63e1525ccd05101000000629e4604fbfe2b256003ed5a (good)
;; QUESTION SECTION:
;ns-b.yourdomain.tld. IN A

;; ANSWER SECTION:
ns-b.yourdomain.tld. 60 IN A 136.144.xxx.xx (edited for privcay, the correct IP address is shown)

;; Query time: 4 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Mon Jun 06 18:23:00 UTC 2022
;; MSG SIZE rcvd: 95

 

Sorry, didn't understand you correclty the first time. Here's the output, command run from the second nameserver querying for the domain that's not working / can not use the nameservers:

# dig @localhost mydomain.tld

; <<>> DiG 9.16.1-Ubuntu <<>> @localhost mydomain.tld
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 18247
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
; COOKIE: 05cab2ab73272c9d01000000629e4c5f92bc9ed2b20483eb (good)
;; QUESTION SECTION:
;mydomain.tld. IN A

;; ANSWER SECTION:
mydomain.tld. 60 IN A 149.210.xxx.xx (shows correct IP address)

;; Query time: 36 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Mon Jun 06 18:50:07 UTC 2022
;; MSG SIZE rcvd: 92

 

Use the same command on the working name server, you see the answer is different.
Something is wrong on second name server, it does not have all the information of the domain / zone. Check name server logs, or ispconfig logs to see why data is not transferred.
My signature has link to DNS setup tutorial.

 

Thanks @Taleman , I'll look into it with that info.