Filesystem Backup after-backup command problem

I have set a post-command to scheduled tar backup in filesystem backup module. I have twice set execute permissions to a script that is run as post-command. Somehow after the script is run it removes execute right (from both user and group) and next time backup failes with "After-backup command failed!" message.

Does this after-backup command somehow change the file permissions or what is the deal here? I certainly don't change the permissions my self. The after-backup script sets group changes group ownership of the backup files.

It has following rows:
#!/bin/bash
# change backup file rights and owners to correct
chown -R root.group /backup
exit 0

This is CentOS 5.