PowerDNS secondary servers receive empty SOA response for particular zone.

[Bob McDonald]

Under certain circumstances, DNS zones representing Windows Active
Directory domains can have rather large numbers of NS records if there
are/were DCs running DNS. This can happen in any DNS zone with a large
number of secondary DNS servers.

The size of the TCP packets is a problem. You might also look at
minimal-responses.

Look at the structure of your DNS environment.

Bob
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