As mentioned in the Announcement for Introducing Pi-hole v6, Pi-hole v6's employment of an embedded webserver has eliminated the need for lighttpd as well as PHP, with the newly embedded webserver leveraging Lua for serving the UI.
Your debug log shows that you run nginx as webserver, with Pi-hole's embedded webserver being completely disabled:
*** [ DIAGNOSING ]: Ports in use
tcp:192.168.42.1:443 is in use by nginx
tcp:0.0.0.0:80 is in use by nginx
tcp:127.0.0.1:443 is in use by nginx
tcp:[::]:80 is in use by nginx
[webserver]
port = "" ### CHANGED, default = "80o,443os,[::]:80o,[::]:443os"
If you want nginx to serve Pi-hole's web UI instead of Pi-hole's embedded webserver, you'd have to add at least Lua capabilities to nginx, and even with Lua support, nginx won't be able to retrieve data from pihole-FTL.
You should enable Pi-hole's embedded webserver on its own port, and perhaps optionally configure nginx as reverse proxy, redirecting HTTP requests to Pi-holes webserver.
To start Pi-hole's webserver on a different port, you'd need to configure webserver.port under All settings » Miscellaneous, or via CLI, e.g. to have it listen on 8081 (IPv4/IPv6 HTTP) and 8443 (IPv4/IPv6 HTTPS):
sudo pihole-FTL --config webserver.port "8081o,8443os,[::]:8081o,[::]:8443os"
All settings is available in Expert mode only.
EDIT:
Unrelated to your webserver issue, there are two other observations from your debug log you may want to address, one of them urgently.
a. consider to disable Pi-hole's embedded NTP service via All settings » Network Time Sync, as your debug log indicates there is already a running NTP service on your machine:
*** [ DIAGNOSING ]: Pi-hole diagnosis messages
count last timestamp type message blob1 blob2
----- ------------------- ------- ------------------------------------------------------------ -------------------- -------
1 2025-02-22 08:54:50 NTP Cannot bind to IPv4 address 0.0.0.0:123 (Address in use), IP Error server
v4 NTP server not available
This isn't critical, as Pi-hole detects the other NTP service and won't enable its own.
b. Your router advertises its own IPv6 addresses as DNS server:
Recursive DNS server 1/2: fd00::<redacted>15
Recursive DNS server 2/2: 2a<redacted>15
This is critical: As your router is advertising its own IPv6 addresses, that would allow your clients to by-pass Pi-hole (which is completely independent of any of Pi-hole's versions).
As your debug log indicates you are using a Fritzbox router, you could consider to configure it to distribute Pi-hole's IPv4 for DNS exclusively, see Unresolved ipv6 adress in my top list - #4 by Bucking_Horn.