Hi, I have similar behaviour, also in a new fresh instalation under armbian (range Pi Zero board).
The situation is also similar abot other guy few month ago in this topic: Pihole running, but can't access admin page - #13 by deHakkelaar
I tried all steps, with the difference that the folder /var/log/lighttpd doesn't esxist, but it is already located in /etc/lighttpd (or I hope)
If I check the status of PiHole, it seems that locally it is working properly, but the admin web access is not accessible, (as your situation).
With this command: journalctl --no-pager -u lighttpd
The response is too long, but seems that the issue comes with: Failed to start Lighttpd Daemon
May 09 01:10:27 orangepizero systemd[1]: lighttpd.service: Main process exited, code=exited, status=255/n/a
May 09 01:10:27 orangepizero systemd[1]: lighttpd.service: Unit entered failed state.
May 09 01:10:27 orangepizero systemd[1]: lighttpd.service: Failed with result 'exit-code'.
May 09 01:10:27 orangepizero systemd[1]: lighttpd.service: Service hold-off time over, scheduling restart.
May 09 01:10:27 orangepizero systemd[1]: Stopped Lighttpd Daemon.
May 09 01:10:27 orangepizero systemd[1]: lighttpd.service: Start request repeated too quickly.
May 09 01:10:27 orangepizero systemd[1]: Failed to start Lighttpd Daemon.
May 09 01:10:27 orangepizero systemd[1]: lighttpd.service: Unit entered failed state.
May 09 01:10:27 orangepizero systemd[1]: lighttpd.service: Failed with result 'exit-code'.
Debug Token: 227xfgucun
Tomorrow I will reinstall all again. (here 1.11 am).
thanks in advance!
SinE80