In your /etc/init.d/pihole-FTL you can find a line
su -s /bin/sh -c "/usr/bin/pihole-FTL" "$FTLUSER"
replace it by
/usr/bin/pihole-FTL
In your /etc/init.d/pihole-FTL you can find a line
su -s /bin/sh -c "/usr/bin/pihole-FTL" "$FTLUSER"
replace it by
/usr/bin/pihole-FTL