Installing and having the error "dns server failure" is a IPV6 error in my case. Fritzbox has enabled IPV6 but my provider has no IPV6 support.
Disabling IPV6 on Fritzbox or disabling IPV6 in Pihole solve this problem!
Use this to deactivate IPV6 in pihole, if IPV6 Adress Reservation is on, then dont use this!
nano /etc/sysctl.conf
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1