There have been some problems with this version of Ubuntu (Redirecting...)
*** [ DIAGNOSING ]: Operating system
[✓] Ubuntu 18.04.1 LTS
Related threads:
Your debug log shows multiple IP addresses which may be contributing:
*** [ DIAGNOSING ]: Networking
[✓] IPv4 address(es) bound to the eth0 interface:
192.168.86.47/24 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)
192.168.86.11/24 matches the IP found in /etc/pihole/setupVars.conf
*** [ DIAGNOSING ]: Ports in use
*:80 lighttpd (IPv4)
*:80 lighttpd (IPv6)
*:22 sshd (IPv4)
*:22 sshd (IPv6)
192.168.86.47:53 pihole-FTL (IPv4)
127.0.0.1:53 pihole-FTL (IPv4)
And this indicates what you are already seeing - cannot resolve DNS from the Pi-Hole IP.
*** [ DIAGNOSING ]: Name resolution (IPv4) using a random blocked domain and a known ad-serving domain
[✓] 6g7ts9dg9q.centade.com is 0.0.0.0 via localhost (127.0.0.1)
[✗] Failed to resolve 6g7ts9dg9q.centade.com via Pi-hole (192.168.86.11)
[✗] Failed to resolve doubleclick.com via a remote, public DNS server (8.8.8.8)