Hmm, in case of any further problem please run
sudo service pihole-FTL stop
sudo killall pihole-FTL
sudo -u pihole pihole-FTL debug
where the first two commands should ensure that no FTL process (in whatever status) is running and the last command should start pihole-FTL in debugging mode (i.e. more verbose output and no switching into daemon mode).
Tell us what you see in this case.