Gravity Update: DNS resolution is currently unavailable

[deHakkelaar]

EDIT: Oh missed below one:

@Buddy

Does this update may have caused the problem?

I wouldnt know as I dont have a Fritzbox.

@Buddy

There is one a line
dns-server: 192:168:???:??
with the right IP RasPi Pi-hole address. There is no line saying
Recursive DNS server.

That looks good.
But isnt 100% proof that no IPv6 DNS server(s) is/are advertised on your LAN.
Were you able to find below router settings without changing any:

@Bucking_Horn

Current FritzBox models should support deactivating IPv6 DNS servers via Home Network|Network|Network Settings|IP Addresses|IPv6 Settings.

If your 7530 is currently handling DHCP and DHCPv6, then you could try to:

a. Untick Also announce DNSv6 server via router advertisement (RFC 5006).
b. Tick Disable DHCPv6 server in the FRITZ!Box for the home network and
b.1. choose There are no other DHCPv6 servers for the home network.

And how are they configured now?

Have you checked on another host how its configured for IPv6 DNS?