I dont see anything wrong.
You could try run my stat command and check if maybe one of the files/folders in the subfolders lack proper permissions/ownership ^^^
What error(s) is/are displayed when tailing the logs live while accessing the webGUI?
sudo tail -F /var/log/lighttpd/{access,error}-pihole.log
You could also try a fresh start:
Nothing wrong with that:
$ ps -o uid,user,gid,group,pid,cmd -C lighttpd
UID USER GID GROUP PID CMD
33 www-data 33 www-data 2609 /usr/sbin/lighttpd -D -f /etc/lighttpd/lighttpd.conf