Lighttpd 500 - Internal Server Error - pihole seems to be working

Raspberry pi3 model b.
OS: Distributor ID:|Raspbian|
Description: Raspbian GNU/Linux 10 (buster)
Release:10
Codename:buster

Actual Behaviour:

[After apt update i get 500 Internal Server Error on web pages but service is still runing.]

lighttpd -tt -f /etc/lighttpd/lighttpd.conf
2021-05-18 18:32:35: (server.c.1493) WARNING: unknown config-key: alias.url (ignored)

pi@raspberrypi:/etc/lighttpd/conf-enabled $ ls
10-fastcgi.conf 15-fastcgi-php.conf 90-javascript-alias.conf

pi@raspberrypi:/etc/lighttpd/conf-enabled $ cat 90-javascript-alias.conf
alias.url += ("/javascript" => "/usr/share/javascript")

Debug Token:

[https://tricorder.pi-hole.net/x4hfyesmsg]

I don't want to reinstall.

Which pages?

The admin interface is working:

[✓] Web interface X-Header: X-Pi-hole: The Pi-hole Web interface is working!

I do see some errors in the logs:

   2021-05-18 18:17:10: (http-header-glue.c.1250) read(): Connection reset by peer 9 10 
   2021-05-18 18:17:10: (gw_backend.c.2149) response not received, request sent: 488 on socket: unix:/var/run/lighttpd/php.socket-0 for /admin/index.php?, closing connection 

My install on Raspbian Buster does not have your 90-javascript-alias.conf file, and has a 99-unconfigured.conf file. Did you add 90-jacascript-alias.conf yourself? Is the error fixed when you remove that file?

No, the admin interface was not working.
90-javascript-alias.conf has been made by the installer, however, i had to unplug the power cable, wait 15 seconds then restart the Raspberry and it was fixed.
I don't know why it happens, but it's the second time i got this error with the web interface.

Not by the Pi-hole installer.

Maybe

That's not on my stock setup:

dan@raspberrypi:~ $ ls /etc/lighttpd/conf-available/
05-auth.conf         10-evhost.conf         10-proxy.conf         10-ssi.conf        11-extforward.conf
10-accesslog.conf    10-expire.conf         10-rewrite.conf       10-ssl.conf        15-fastcgi-php.conf
10-cgi.conf          10-fastcgi.conf        10-rrdtool.conf       10-status.conf     90-debian-doc.conf
10-dir-listing.conf  10-flv-streaming.conf  10-simple-vhost.conf  10-userdir.conf    99-unconfigured.conf
10-evasive.conf      10-no-www.conf         10-sockproxy.conf     10-usertrack.conf  README
dan@raspberrypi:~ $ ls /etc/lighttpd/conf-enabled/
10-fastcgi.conf  15-fastcgi-php.conf  99-unconfigured.conf
dan@raspberrypi:~ $ apt-cache policy lighttpd
lighttpd:
  Installed: 1.4.53-4+deb10u1
  Candidate: 1.4.53-4+deb10u1
  Version table:
 *** 1.4.53-4+deb10u1 500
        500 http://raspbian.raspberrypi.org/raspbian buster/main armhf Packages
        100 /var/lib/dpkg/status
dan@raspberrypi:~ $ dpkg -S 90-javascript-alias.conf
dpkg-query: no path found matching pattern *90-javascript-alias.conf*

dan@raspberrypi:~ $ dpkg-query -S 90-javascript-alias.conf
dpkg-query: no path found matching pattern *90-javascript-alias.conf*

Yeah I always install apt-file if I know I need to search repos.
There seems to be some packages that depend on it:

pi@ph5b:~ $ apt rdepends javascript-common | grep Depends

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

Reverse Depends:
  Depends: cacti
  Depends: zoneminder
  Depends: shaarli
  Depends: routino-www
  Depends: ocsinventory-reports
  Depends: node-vue
  Depends: libjs-simple-fmt
  Depends: libjs-popper.js
  Depends: libjs-lodash
  Depends: libjs-jquery-mousewheel
  Depends: libjs-jed
  Depends: libjs-extjs
  Depends: libjs-dropzone
  Depends: libjs-chosen
  Depends: libjs-autolink
  Depends: fusiondirectory
  Depends: freedombox
  Depends: dolibarr
  Depends: dokuwiki

But oc any howto or installer could have installed that package.
Or maybe it came with one of those distro's derived from Raspbian.

This seems like a reoccurring thing and would be nice to know where that javascript-common package is coming from.
@paolantoni, did you flash the official Raspbian distro from the Raspberry Pi site or another?
And did you install anything else besides Pi-hole like for example Plex?