Tried to repair the installation with pihole -r, but it's aborting and the point where it restarts lighttpd:
[✓] Root user check
.;;,.
.ccccc:,.
:cccclll:. ..,,
:ccccclll. ;ooodc
'ccll:;ll .oooodc
.;cll.;;looo:.
.. ','.
.',,,,,,'.
.',,,,,,,,,,.
.',,,,,,,,,,,,....
....''',,,,,,,'.......
......... .... .........
.......... ..........
.......... ..........
......... .... .........
........,,,,,,,'......
....',,,,,,,,,,,,.
.',,,,,,,,,'.
.',,,,,,'.
..'''.
[i] Existing PHP installation detected : PHP version 7.3.10-1+0~20191008.45+debian9~1.gbp365209
[i] Repair option selected
[✓] Disk space check
[✓] Update local cache of available packages
[✓] Checking apt-get for upgraded packages... up to date!
[i] Installer Dependency checks...
[✓] Checking for apt-utils
[✓] Checking for dialog
[✓] Checking for debconf
[✓] Checking for dhcpcd5
[✓] Checking for git
[✓] Checking for iproute2
[✓] Checking for whiptail
[i] Performing reconfiguration, skipping download of local repos
[✓] Resetting repository within /etc/.pihole...
[✓] Resetting repository within /var/www/html/admin...
[i] Main Dependency checks...
[✓] Checking for cron
[✓] Checking for curl
[✓] Checking for dnsutils
[✓] Checking for iputils-ping
[✓] Checking for lsof
[✓] Checking for netcat
[✓] Checking for psmisc
[✓] Checking for sudo
[✓] Checking for unzip
[✓] Checking for wget
[✓] Checking for idn2
[✓] Checking for sqlite3
[✓] Checking for libcap2-bin
[✓] Checking for dns-root-data
[✓] Checking for resolvconf
[✓] Checking for libcap2
[✓] Checking for lighttpd
[✓] Checking for php7.3-common
[✓] Checking for php7.3-cgi
[✓] Checking for php7.3-sqlite3
[✓] Enabling lighttpd service to start on reboot...
[i] FTL Checks...
[✓] Detected ARM-hf architecture (armv7+)
[i] Checking for existing FTL binary...
[i] Latest FTL Binary already installed (v4.3.1). Confirming Checksum...
[i] Checksum correct. No need to download!
[✓] Checking for user 'pihole'
[✓] Installing scripts from /etc/.pihole
[i] Installing configs from /etc/.pihole...
[i] Existing dnsmasq.conf found... it is not a Pi-hole file, leaving alone!
[✓] Copying 01-pihole.conf to /etc/dnsmasq.d/01-pihole.conf
[i] Installing blocking page...
[✓] Creating directory for blocking page, and copying files
[✗] Backing up index.lighttpd.html
No default index.lighttpd.html file found... not backing up
[✓] Installing sudoer file
[✓] Installing latest Cron script
[✓] Installing latest logrotate script
[i] Backing up /etc/dnsmasq.conf to /etc/dnsmasq.conf.old
[i] man not installed
[i] Testing if systemd-resolved is enabled
[i] Systemd-resolved is not enabled
[i] Restarting lighttpd service...
Error is the same. That's very weird.