Do you have permission to access the directory itself? Please show the results of:
$ ls -la /mnt
$ ls -la /mnt/data
hts needs read and write permission all the way down from the top.
PS as Jonas might have meant, it's unusual to have a permanent mount point under /mnt - it's usually used for temporary mounts. Perhaps a new directory under /var/lib/tvheadend might make sense as a mount point?
