I have similar symptoms. Only I get a 404 iso 403.
Uninstalling and re-installing seems to work. However, as OP I too would like to run the install unattended.
While re-installing I noticed Cloning https://github.com/pi-hole/AdminLTE.git flying passed.
Could that be the problem?
My first install was using the basic-install.sh --unattended from git clone https://github.com/pi-hole/pi-hole.git.
That doesn't pull in the AdminLTE.git or does it?
It all goes belly up over here
The additional repo(s) need to be cloned here, but when the --unattended option is passed the variable useUpdateVars is made false and in line 1412 it gets decided that the repos don't need cloning.
It is part of the installation process to clone the AdminLTE repo (it's the web interface).
An unattended install assumes you have already specified installation options in /etc/pihole/setupVars.conf. It is not possible to install without prompts unless you either have already chosen settings or we choose settings for you.
I Install the OS on a clean SD-card using an unattended installer on a headless Raspberry Pi
I then have several scripts that run automagically at first boot and post-install various packages (also unattended). Part of the unattended installation is to mkdir -p /etc/pihole and scp my version of setupVars from local network storage.
Next, the script git clones the Pi-hole repo.
At this point I would like to also be able to do an unattended installation of Pi-hole on my headless Pi.
So, it would be extremely nice if the entire installation could be done unattended on a virgin headless system.
I've not actually run the unattended install for a while, but you should just be able to pre-populate a setupvars.conf and go from there... BRB, trying it...
Freshly flashed SD card, sudo raspi-config run to expand the filesystem, and not a lot else. Maybe an apt-get update && apt-get upgrade chucked in there for good measure.
If useUpdateVars is true (as it is in this case) it will hit L1450 instead.