problems changing NS records

[Lucio Crusca]

Until a few hours ago, I had several domains and 3 nameservers for them:

ns1.virtualbit.it (master, 136.243.232.142)
ns11.virtualbit.it (slave, 158.69.210.19)
ns2.virtualbit.it (slave, 136.243.232.143)

Then I tried to migrate to a new master, names.virtualbit.it (46.4.38.130).
Here is the migration steps I followed:

1) I configured Bind on the new server as master for all domains
2) I added the master IP address to the two slaves with:

masters vbitdnsmasters { 136.243.232.142; 46.4.38.130; };

zone "acquaritalia.it" {
        type slave;
        masters { vbitdnsmasters; };
        file "/var/lib/bind/acquaritalia.it.db";
};

and so on for all other zones.

3) I updated the NS records in the zone files, and in the control panel of
the domain registrar

Now all the domains have problems. IntoDNS reports:

"
Nameservers A records    ERROR: Some of your DNS servers do not have A
records at all. I could not find any A records for the following DNS
servers:
ns2.virtualbit.it

You must have A records for all of your nameservers.
"

However:

dig @136.243.232.143 -t ns acquaritalia.it
dig @136.243.232.143 acquaritalia.it
dig @136.243.232.143 www.acquaritalia.it
dig @136.243.232.143 -t mx acquaritalia.it

all work as expected. Meanwhile, the same queries against 8.8.8.8 sometime
work, sometime reply SERVFAIL.

I don't know what I did wrong. Please help.
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