[Solved!] The NS server periodically stops responding

Hi!
I have a server with a Hestia control panel.
I use e3i.ru_ domain as my primary domain and ns1.e3i.ru_ and ns2.e3i.ru_ its NS records (child-ns mode).
Other domains, for example razvitieterritorii.ru_ and litzenwaerme.ru_ also use child-ns mode.
When checking the NS records 2whois.ru - сервис NSLOOKUP: информация о домене "razvitieterritorii.ru"
service shows that records or the server, periodically unavailable.

Also A records don’t work for some domains - webmail, pop etc… , but they work for others. (For example webmail.e3i.ru_ does not work)
Replication works poorly for domain razvitieterritorii.ru, but for litzenwaerme.ru_ everything is fine.

What am I doing wrong?

Linux e3i.ru_ 4.19.0-17-amd64 #1 SMP Debian 4.19.194-3 (2021-07-18) x86_64

Hestia
core package 1.5.4 (amd64)
php interpreter 7.4.27-1 (amd64)
hestia-nginx 1.21.5 (amd64)

Solved!
Brutforce monitor fail2ban was blocking connections to the server.

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