Neither of the IPv6 addresses assigned to the Pi match the IPv6 in the Pi-Hole setup. I altered the IPv6 address for posting purposes - your debug log has the specifics.
[✓] IPv6 address(es) bound to the eth0 interface:
xxxx....:7d7f:9a41 does not match the IP found in /etc/pihole/setupVars.conf (https://discourse.pi-hole.net/t/use-ipv6-ula-addresses-for-pi-hole/2127)
fe80::800f:2c5a:ed05:29e1 does not match the IP found in /etc/pihole/setupVars.conf (https://discourse.pi-hole.net/t/use-ipv6-ula-addresses-for-pi-hole/2127)
[i] Default IPv6 gateway: fe80::464e:6dff:fed8:37d5
fe80::464e:6dff:fed8:37d5
* Pinging fe80::464e:6dff:fed8:37d5
fe80::464e:6dff:fed8:37d5...
[✗] Gateway did not respond. (https://discourse.pi-hole.net/t/why-is-a-default-gateway-important-for-pi-hole/3546)
*** [ DIAGNOSING ]: Setup variables
PIHOLE_INTERFACE=eth0
IPV4_ADDRESS=192.168.178.5/24
IPV6_ADDRESS=xxxx....:6bff:fe8f
If you don't specifically need IPv6 on your network (and very few users do), then I would disable it on the network and avoid potential IPv6 problems. The IPv6 DNS queries can still be handled by IPv4.