It's not possible to know what's happening on your system using the information on the debug log, since you are using a custom installation.
The web interface is not installed on the default location (your debug log says it's not installed):
*** [ DIAGNOSING ]: Web version
Directory /var/www/html/admin doesn't exist
Also, you are not using the default webserver (lighttpd) and there are no logs. You are using Apache.
*** [ DIAGNOSING ]: contents of /etc/lighttpd
/etc/lighttpd does not exist.
*** [ DIAGNOSING ]: contents of /var/log/lighttpd
/var/log/lighttpd does not exist.
My pihole was working well for more than a year with apache. Dashboard and everything worked perfectly during this period until now.
This problem happened only since few days.
[✓] Update local cache of available packages
[i] Existing PHP installation detected : PHP version 8.1.14
[✓] Checking apt-get for upgraded packages... up to date!
[i] Checking for / installing Required dependencies for OS Check...
[✓] Checking for grep
[✓] Checking for dnsutils
[✓] Supported OS detected
[i] Checking for / installing Required dependencies for this install script...
[✓] Checking for git
[✓] Checking for iproute2
[✓] Checking for whiptail
[✓] Checking for ca-certificates
[i] SELinux not detected
[i] Repair option selected
[i] Performing reconfiguration, skipping download of local repos
[✓] Resetting repository within /etc/.pihole...
Unable to reset /var/www/html/admin, exiting installer
Pi-hole installer expects the web interface is installed on /var/www/html/admin/.
If you changed this, the Debug Log won't find the web interface, but is not a big issue.
The problem here is:
you are trying to repair your custom installation using pihole -r, but this command expects to find a directory /var/www/html/admin/ and the path doesn't exist.
I don't know why, but I saw you are using older versions of every Pi-hole component:
Pi-hole core: v5.9 (current is v5.14.2)
FTL: v5.14 (current is v5.20)
Web interface: unknown (current is v5.18)
My suggestion is to reinstall Pi-hole to use the current versions. I think you can keep the web interface on /var/www/html/admin/ and change Apache config.