Yeah actually it seems that permissions is indeed the issue... If I change the user and group inside the start-stop script then it works.
The thing is that I ran the chown tvheadend-testing:users tv_grab_file command as well as chmod 777 tv_grab_file but it doesn't seem to have helped.
Am I missing something?
Solved it: The issue was the folder that the file was laying in... I used a symlink but the tvheadend-testing user didn't have access to the origin folder that the symlink was pointing to. I gave read/write permissions to the user tvheadend-testing through DSM and it worked.