how are you calling the script?
edit the /etc/crontab
and add the line:
40 3 * * * /etc/back-res 1>/dev/null 2>/dev/null
This will call the script at 40 minutes past three each day and will send any error messages to the bit bin.
Check to see if your script is in fact making the back up tars in your designated directory regardless of the errors it's giving. I understand some errors aren't critical.