Put the -C into TVH_ARGS in the config file /etc/default/tvheadend, i.e.
# TVH_ARGS
# add any other arguments
TVH_ARGS="-C"
This is brutal, though, as it switches off access control but doesn't fix the problem. If you need to edit the config files, do it as hts (I'm assuming here that all is installed as a normal 'buntu package):
sudo su hts
<your password>
I'm just wondering what would happen if you:
1. Shut down tvheadend:
sudo service tvheadend stop
<your password if prompted>
2. Remove the access control files:
sudo su hts
<your password if prompted>
cd ~/.hts/tvheadend/accesscontrol
mkdir /tmp/tvheadendbackup
mv * /tmp/tvheadendbackup
exit
(if something goes wrong, you can restore the access control rules since you have a backup)
3. Reconfigure the package to reset your superuser credentials:
sudo dpkg-reconfigure tvheadend
<provide new credentials>
4. Restart
sudo tvheadend start
5. Clear your browser cache and cookies related to tvheadend
6. Navigate to the web page and log in with the details from (3)