PID file should go to /run, not /var/run
Per systemd report:
Dec 03 16:21:07 ... systemd[1]: /usr/lib/systemd/system/lighttpd.service:6:
PIDFile= references a path below legacy directory /var/run/, updating
/var/run/lighttpd.pid → /run/lighttpd.pid; please please update the unit
file accordingly.