Web admin page won't show up, index.php error

there is only one line in there:
alias.url += ("/javascript" => "/usr/share/javascript")

This is a default:

dan@raspberrypi:/etc/lighttpd/conf-enabled $ ls -la
total 8
drwxr-xr-x 2 root root 4096 Oct 19 00:31 .
drwxr-xr-x 4 root root 4096 Oct 19 00:31 ..
lrwxrwxrwx 1 root root   33 Oct 19 00:31 10-fastcgi.conf -> ../conf-available/10-fastcgi.conf
lrwxrwxrwx 1 root root   37 Oct 19 00:31 15-fastcgi-php.conf -> ../conf-available/15-fastcgi-php.conf
lrwxrwxrwx 1 root root   38 Oct 19 00:30 99-unconfigured.conf -> ../conf-available/99-unconfigured.conf
dan@raspberrypi:/etc/lighttpd/conf-enabled $ cat *
--10-fastcgi.conf
# /usr/share/doc/lighttpd/fastcgi.txt.gz
# http://redmine.lighttpd.net/projects/lighttpd/wiki/Docs:ConfigurationOptions#mod_fastcgi-fastcgi

server.modules += ( "mod_fastcgi" )

--15-fastcgi-php.conf
# -*- depends: fastcgi -*-
# /usr/share/doc/lighttpd/fastcgi.txt.gz
# http://redmine.lighttpd.net/projects/lighttpd/wiki/Docs:ConfigurationOptions#mod_fastcgi-fastcgi

## Start an FastCGI server for php (needs the php5-cgi package)
fastcgi.server += ( ".php" =>
        ((
                "bin-path" => "/usr/bin/php-cgi",
                "socket" => "/var/run/lighttpd/php.socket",
                "max-procs" => 1,
                "bin-environment" => (
                        "PHP_FCGI_CHILDREN" => "4",
                        "PHP_FCGI_MAX_REQUESTS" => "10000"
                ),
                "bin-copy-environment" => (
                        "PATH", "SHELL", "USER"
                ),
                "broken-scriptfilename" => "enable"
        ))
)

--99-unconfigured.conf
# override prior index-file.name directive
# to fall back to default index.lighttpd.html
index-file.names := ( "index.php", "index.html", "index.lighttpd.html" )

should I ln that file here?

I checked the other two files. They are the same as your output.

First find out what put that extra config in, I don't have a javascript redirect in /etc/lighttpd/conf-available so it's not in the stock Raspbian setup.

That's malformed anyways.

Edit:
Run

lighttpd -p -f /etc/lighttpd/lighttpd.conf

and post up the output.

Here it is:

config {
    var.PID                        = 10206
    var.CWD                        = "/home/pi"
    mimetype.assign                = (
        ".ico"   => "image/x-icon",
        ".jpeg"  => "image/jpeg",
        ".jpg"   => "image/jpeg",
        ".png"   => "image/png",
        ".svg"   => "image/svg+xml",
        # 5
        ".css"   => "text/css; charset=utf-8",
        ".html"  => "text/html; charset=utf-8",
        ".js"    => "text/javascript; charset=utf-8",
        ".json"  => "application/json; charset=utf-8",
        ".map"   => "application/json; charset=utf-8",
        # 10
        ".txt"   => "text/plain; charset=utf-8",
        ".eot"   => "application/vnd.ms-fontobject",
        ".otf"   => "font/otf",
        ".ttc"   => "font/collection",
        ".ttf"   => "font/ttf",
        # 15
        ".woff"  => "font/woff",
        ".woff2" => "font/woff2",
        # 17
    )
    server.document-root           = "/var/www/html"
    server.error-handler-404       = "/pihole/index.php"
    server.upload-dirs             = ("/var/cache/lighttpd/uploads")
    server.errorlog                = "/var/log/lighttpd/error.log"
    server.pid-file                = "/run/lighttpd.pid"
    server.username                = "www-data"
    server.groupname               = "www-data"
    server.port                    = 80
    accesslog.filename             = "/var/log/lighttpd/access.log"
    accesslog.format               = "%{%s}t|%V|%r|%s|%b"
    index-file.names               = ("index.php", "index.html", "index.lighttpd.html")
    url.access-deny                = ("~", ".inc", ".md", ".yml", ".ini")
    static-file.exclude-extensions = (".php", ".pl", ".fcgi")
    compress.cache-dir             = "/var/cache/lighttpd/compress/"
    compress.filetype              = (
        "application/json",
        "application/vnd.ms-fontobject",
        "application/xml",
        "font/eot",
        "font/opentype",
        "font/otf",
        "font/ttf",
        "image/bmp",
        "image/svg+xml",
        "image/vnd.microsoft.icon",
        "image/x-icon",
        "text/css",
        "text/html",
        "text/javascript",
        "text/plain",
        "text/xml",
        # 16
    )
    server.modules                 = (
        "mod_access",
        "mod_accesslog",
        "mod_auth",
        "mod_expire",
        "mod_compress",
        "mod_redirect",
        "mod_setenv",
        "mod_rewrite",
        "mod_fastcgi",
        # 9
    )
    alias.url                      = (
        "/javascript" => "/usr/share/javascript",
    )
    fastcgi.server                 = (
        ".php" => (
            (
                "bin-path"              => "/usr/bin/php-cgi",
                "socket"                => "/var/run/lighttpd/php.socket",
                "max-procs"             => 1,
                "bin-environment"       => (
                    "PHP_FCGI_CHILDREN"     => "4",
                    "PHP_FCGI_MAX_REQUESTS" => "10000",
                    # 2
                ),
                "bin-copy-environment"  => ("PATH", "SHELL", "USER"),
                # 5
                "broken-scriptfilename" => "enable",
                # 6
            ),
        ),
    )
    expire.url                     = (
        "" => "access plus 0 seconds",
    )


    $SERVER["socket"] == "[::]:80" {
        # block 1

    } # end of $SERVER["socket"] == "[::]:80"

    $HTTP["url"] =~ "^/admin/" {
        # block 2
        setenv.add-response-header = (
            "X-Pi-hole"       => "The Pi-hole Web interface is working!",
            "X-Frame-Options" => "DENY",
            # 2
        )


        $HTTP["url"] =~ "\.(eot|otf|tt[cf]|woff2?)$" {
            # block 3
            setenv.add-response-header = (
                "Access-Control-Allow-Origin" => "*",
            )

        } # end of $HTTP["url"] =~ "\.(eot|otf|tt[cf]|woff2?)$"
    } # end of $HTTP["url"] =~ "^/admin/"

    $HTTP["url"] =~ "^/admin/\.(.*)" {
        # block 4
        url.access-deny = ("")

    } # end of $HTTP["url"] =~ "^/admin/\.(.*)"
}

Okay, and does

curl http://127.0.0.1/admin/

run all the way to </html>?

Nothing comes out from running this.....

Okay,

curl -I http://127.0.0.1/admin/

here it is:
HTTP/1.1 500 Internal Server Error
Content-type: text/html; charset=UTF-8
X-Pi-hole: The Pi-hole Web interface is working!
X-Frame-Options: DENY
Date: Sun, 01 Nov 2020 05:22:17 GMT
Server: lighttpd/1.4.53

Remove that 90-javascript file.

rm 90-javascript-alias.conf
from conf-enabled?

sudo lighty-disable-mod 90-javascript-alias

Ignoring unknown module: 90-javascript-alias
Run "service lighttpd force-reload" to enable changes

pi@raspberrypi:~ $ service lighttpd force-reload
==== AUTHENTICATING FOR org.freedesktop.systemd1.manage-units ===
Authentication is required to reload 'lighttpd.service'.
Multiple identities can be used for authentication:

  1. ,,, (pi)
  2. root
    Choose identity to authenticate as (1-2): 1
    Password:
    ==== AUTHENTICATION COMPLETE ===

That file then is unknown and something put it there without lighttpd's knowledge.

sudo rm that file and then sudo systemctl restart lighttpd after the file is gone.

pi@raspberrypi:/etc/lighttpd/conf-enabled $ sudo rm 90-javascript-alias.conf
pi@raspberrypi:/etc/lighttpd/conf-enabled $ sudo systemctl restart lighttpd

Is it working now?

pi@raspberrypi:~ $ curl -i http://127.0.0.1/admin/
HTTP/1.1 500 Internal Server Error
Content-type: text/html; charset=UTF-8
X-Pi-hole: The Pi-hole Web interface is working!
X-Frame-Options: DENY
Content-Length: 0
Date: Sun, 01 Nov 2020 05:32:39 GMT
Server: lighttpd/1.4.53

That's all I can think of, this is just a stock Raspbian?

cat /etc/os-release

and check apt update to see if there are any updates to come in.

I don't know what/why that extra config file was installed.

Edit: How did you install Plex?

PRETTY_NAME="Raspbian GNU/Linux 10 (buster)"
NAME="Raspbian GNU/Linux"
VERSION_ID="10"
VERSION="10 (buster)"
VERSION_CODENAME=buster
ID=raspbian
ID_LIKE=debian
HOME_URL="http://www.raspbian.org/"
SUPPORT_URL="RaspbianForums - Raspbian"
BUG_REPORT_URL="RaspbianBugs - Raspbian"