If running as user "hts", then Tvheadend will not have permission to create the log file in /var/log.
You can try first as root:
touch /var/log/tvheadend.log
chmod 666 /var/log/tvheadend.log
Then Tvheadend will be able to write to it.
Or, put the log file someplace user "hts" will always have permission, like /home/hts.