403 forbiden on admin page

Keep in mind though that with the v6 release, the lighttpd dependency is dropped and replaced by a web daemon embedded in the pihole-FTL binary/daemon:

dehakkelaar@ph6b:~$ pihole-FTL --config webserver
webserver.domain = pi.hole
webserver.acl =
webserver.port = 80,[::]:80,443s,[::]:443s
webserver.session.timeout = 300
webserver.session.restore = true
webserver.tls.cert = /etc/pihole/tls.pem
webserver.paths.webroot = /var/www/html
webserver.paths.webhome = /admin/
webserver.interface.boxed = true
webserver.interface.theme = default-auto
webserver.api.max_sessions = 16
webserver.api.prettyJSON = false
webserver.api.pwhash =
webserver.api.password = ********
webserver.api.app_pwhash =
webserver.api.app_sudo = false
webserver.api.cli_pw = true
webserver.api.excludeClients = []
webserver.api.excludeDomains = []
webserver.api.maxHistory = 86400
webserver.api.maxClients = 10
webserver.api.client_history_global_max = true
webserver.api.allow_destructive = true
webserver.api.temp.limit = 60.000000
webserver.api.temp.unit = C
dehakkelaar@ph6b:~$ sudo ss -nltup | grep 'Netid\|pihole-FTL'
Netid State  Recv-Q Send-Q Local Address:Port Peer Address:PortProcess
udp   UNCONN 0      0            0.0.0.0:53        0.0.0.0:*    users:(("pihole-FTL",pid=921351,fd=16))
udp   UNCONN 0      0            0.0.0.0:123       0.0.0.0:*    users:(("pihole-FTL",pid=921351,fd=34))
udp   UNCONN 0      0               [::]:53           [::]:*    users:(("pihole-FTL",pid=921351,fd=18))
udp   UNCONN 0      0               [::]:123          [::]:*    users:(("pihole-FTL",pid=921351,fd=35))
tcp   LISTEN 0      200          0.0.0.0:443       0.0.0.0:*    users:(("pihole-FTL",pid=921351,fd=29))
tcp   LISTEN 0      32           0.0.0.0:53        0.0.0.0:*    users:(("pihole-FTL",pid=921351,fd=17))
tcp   LISTEN 0      200          0.0.0.0:80        0.0.0.0:*    users:(("pihole-FTL",pid=921351,fd=27))
tcp   LISTEN 0      200             [::]:443          [::]:*    users:(("pihole-FTL",pid=921351,fd=30))
tcp   LISTEN 0      32              [::]:53           [::]:*    users:(("pihole-FTL",pid=921351,fd=19))
tcp   LISTEN 0      200             [::]:80           [::]:*    users:(("pihole-FTL",pid=921351,fd=28))