I've experience similar permission issues and couldn't figure out how to fix them. Found the steps below from another site.
This is not the most secure approach, but it does solve the problem.
SSH to synology server as root
Change to config file directory
Create a backup of the config file
cp SickBeard.sh SickBeard.sh.backup
Use vi to edit SickBeard.sh
Change the line starting with "su" from the sickbeard user to your root username and add your password:
su root password -s /bin/sh -c . . . .
Note: Simple vi commands to help you make the change:
*i Insert text
:wq quit and save
esc to enter “command mode”
Reboot and check post processing again. Worked for me. . .
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