Hello again @gani .
Managed to get this working on my own, and finally i can get to the web UI with all functions.
Simply was a matter of editing the /etc/sudoers.d/bacularis with the proper path.
Than, logged into the https://myserver:9097/config/panel and reconfigured the acces.
Rebooted and logged in with previous working credentials.
Ran a backup copy of a random client, just to be sure. Worked 100%.
A short follow-up of the whole process;
I added the Ubuntu 24.04 repositories to an existing Ubuntu 22.04 system, then updated the bacularis and bacularis-lighttpd packages, rebooted and verified that Bacularis was still working properly.
Started the upgrade from 22.04 to 24.04.
During the process, I noticed messages indicating that PHP 8.3 packages were removed and several third-party repositories were disabled.
Once the upgrade finished, I rebooted the system.
After the upgrade, i tried accessing the previous Bacularis web URL — it now returned a 404 Not Found error (expected due to missing PHP support).
Reinstalled the PHP 8.3 packages:
sudo apt install php8.3 php8.3-fpm php8.3-cli php8.3-curl php8.3-xml php8.3-mbstring php8.3-bcmath php8.3-zip php8.3-gd php8.3-intl
Also, i edited /etc/bacularis/bacularis-lighttpd.conf to update:
"socket" => "/run/php/php8.3-fpm.sock"
(previously pointed to php8.1-fpm.sock)
Restarted the PHP-FPM service and tried again accessing the Bacularis web URL, realized that i needed to
reinstalled the bacularis and bacularis-lighttpd packages again and this also reinstalled necessary PHP dependencies.
The Bacularis login page was now accessible, but login attempts returned: “Invalid user or password”
Installed apache2-utils to be able to use htpasswd and create a new user, so i created it:
sudo htpasswd /etc/bacularis/Web/bacularis.users newuser
Edited the API User Authentication configuration file:
sudo nano /etc/bacularis/API/api.conf
like this
[api]
auth_type = "basic"
debug = "0"
lang = "en"
user_file = "/usr/share/bacularis/protected/vendor/bacularis/bacularis-api/API/Config/bacularis.users"
With that, I was finally able to authenticate and access the Bacularis UI, but still some issues were there:

Edited the sudoers file to allow www-data to execute bconsole, from the correct path
sudo nano /etc/sudoers.d/bacularis
and added the line "www-data ALL=NOPASSWD: /opt/bacula/bin/bconsole"
Verified with
sudo -u www-data sudo /opt/bacula/bin/bconsole -t
returned no errors.
Connected to https://myserver:9097/config/panel and ran the conmfiguration wizard again. Restarted both services for good measure
sudo systemctl restart bacularis-lighttpd php8.3-fpm
Successfully logged in with previous credentials and everything now works as expected.
Hope that helps somebody outhere.
PS @gani hope that i didn't screwed up the API User Authentication logic with adding the user_file = "/usr/share/bacularis/protected/vendor/bacularis/bacularis-api/API/Config/bacularis.users" in the /etc/bacularis/API/api.conf
Thanks !