YES! That was it! Actually I ran into two problems (Debian Bookworm amd64 4.3-2321 installation from the .deb package on cloudsmith.io):
1) the installer created an empty superuser file after i inserted username and password:
$ sudo cat /var/lib/tvheadend/superuser
{
}
2) the location was wrong as by your post. So the fix was
$ sudo systemctl stop tvheadend
$ sudo mv /var/lib/tvheadend/superuser /var/lib/tvheadend/.config/hts/superuser
(edit /var/lib/tvheadend/.config/hts/superuser and insert "username": "XXX", "password": "YYY")
$ sudo systemctl start tvheadend
Then i managed to enter and reach the config panel.