And can you access the Pi-hole admin panel again now? Otherwise check again:
systemctl status lighttpd
tail -20 /var/log/lighttpd/error.log
EDIT: Okay that config backup was not created by the Pi-hole installed, as it would then have
.orig ending. Also
dietpi-software does not create a backup of it but only does a single change to it (the document root). Check whether everything can be accessed again, also Nextcloud and phpMyAdmin, else probably that is a backup from before the DietPi software install and the following would then fix it:
G_SUDO G_CONFIG_INJECT 'server.document-root' 'server.document-root = "/var/www"' /etc/lighttpd/lighttpd.conf
# Required for Nextcloud:
G_SUDO G_CONFIG_INJECT '"mod_access",' ' "mod_access",' /etc/lighttpd/lighttpd.conf '"mod_.+",'
# Required for Pi-hole and Nextcloud:
G_SUDO G_CONFIG_INJECT '"mod_setenv",' ' "mod_setenv",' /etc/lighttpd/lighttpd.conf '"mod_.+",'
sudo systemctl restart lighttpd