Can you run it as su with the command above, please - I don't know if Manjaro has the equivalent of sudo, so it might be su to root then 'tvheadend -f -u hts -g video'.
The -C is specifically:
-C, --firstrun If no user account exists then create one with
no username and no password. Use with care as
it will allow world-wide administrative access
to your Tvheadend installation until you edit
the access-control from within the Tvheadend UI
... so you shouldn't need that normally.
Once running, can you give me 'ps -eaf | grep tvheadend', please.
Also, su to hts and then check group membership with 'groups'.
While you're there, check that /home/hts/.hts/tvheadend/superuser exists and see what it includes - that's your main admin user (unless -C has killed it). Also, check permissions: if you've run tvheadend as root initially then the config files won't be properly accessible to hts. Most things in your config directory should be hts:video, superuser file is hts:hts.
And if there's a config directory in the root user's home directory, rename it to get it out of the way as well to make sure that's not being used.
