The issue I am facing:
I've used pihole for years as DNS filter. Since last upgrade, when a host make a lot of dns queries in the unit time, after a while he gets "REFUSED" when trying to resolve.
My problem is that the host is an http proxy server with ~ 2000 clients and so it has to do tons of DNS queries: this problems occurs only since last update:

Pi-hole v5.3.1

Details about my system:
The host providing DNS filtering is an ubuntu 18.04.

Thanks in advance for any help: I really need it to get back working!
What I have changed since installing Pi-hole:
Nothing, just upgraded.

Your observation matches the rate limiting behaviour as documented in the release notes for Pi-hole FTL v5.7 and Web v5.4 released.

For configuration options, please study those release notes and refer to Pi-hole's RATE_LIMIT documentation.

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