First I used the port. I switched it multiple times during debug and trying, also because I got this everytime:
[1736517641] unbound[7413:0] error: can't bind socket: Address already in use for 127.0.0.1 port 51234 (len 16)
Complete unbound config:
pi@pihole:~ $ sudo grep -v '#\|^$' -R /etc/unbound/unbound.conf*
/etc/unbound/unbound.conf:include-toplevel: "/etc/unbound/unbound.conf.d/*.conf"
/etc/unbound/unbound.conf.d/pi-hole.conf:server:
/etc/unbound/unbound.conf.d/pi-hole.conf: logfile: "/var/log/unbound/unbound.log"
/etc/unbound/unbound.conf.d/pi-hole.conf: verbosity: 4
/etc/unbound/unbound.conf.d/pi-hole.conf: interface: 127.0.0.1
/etc/unbound/unbound.conf.d/pi-hole.conf: port: 5335
/etc/unbound/unbound.conf.d/pi-hole.conf: do-ip4: yes
/etc/unbound/unbound.conf.d/pi-hole.conf: do-udp: yes
/etc/unbound/unbound.conf.d/pi-hole.conf: do-tcp: yes
/etc/unbound/unbound.conf.d/pi-hole.conf: do-ip6: yes
/etc/unbound/unbound.conf.d/pi-hole.conf: prefer-ip6: yes
/etc/unbound/unbound.conf.d/pi-hole.conf: root-hints: "/var/lib/unbound/root.hints"
/etc/unbound/unbound.conf.d/pi-hole.conf: harden-glue: yes
/etc/unbound/unbound.conf.d/pi-hole.conf: harden-dnssec-stripped: yes
/etc/unbound/unbound.conf.d/pi-hole.conf: use-caps-for-id: no
/etc/unbound/unbound.conf.d/pi-hole.conf: edns-buffer-size: 1232
/etc/unbound/unbound.conf.d/pi-hole.conf: prefetch: yes
/etc/unbound/unbound.conf.d/pi-hole.conf: num-threads: 1
/etc/unbound/unbound.conf.d/pi-hole.conf: so-rcvbuf: 1m
/etc/unbound/unbound.conf.d/pi-hole.conf: private-address: 192.168.0.0/16
/etc/unbound/unbound.conf.d/pi-hole.conf: private-address: 169.254.0.0/16
/etc/unbound/unbound.conf.d/pi-hole.conf: private-address: 172.16.0.0/12
/etc/unbound/unbound.conf.d/pi-hole.conf: private-address: 10.0.0.0/8
/etc/unbound/unbound.conf.d/pi-hole.conf: private-address: fd00::/8
/etc/unbound/unbound.conf.d/pi-hole.conf: private-address: fe80::/10
/etc/unbound/unbound.conf.d/remote-control.conf:remote-control:
/etc/unbound/unbound.conf.d/remote-control.conf: control-enable: yes
/etc/unbound/unbound.conf.d/remote-control.conf: control-interface: /run/unbound.ctl
/etc/unbound/unbound.conf.d/root-auto-trust-anchor-file.conf:server:
/etc/unbound/unbound.conf.d/root-auto-trust-anchor-file.conf: auto-trust-anchor-file: "/var/lib/unbound/root.key"
I have found a thread with a similiar problem: Unbound times out with: "communications error to 127.0.0.1#5335: timed out"
But it doesn't seem that this got fixed by the guys