it seems that all the backup works perfectly but it fails when the generated zip file is going to be copied to its final destination.
There are two possibilities:
1) If the "backup_sche_destination" in your server is empty, then Moodle will store backup files inside each course, in one "backupdata" directory. If this is your case, you must check that all those course directories are writeable by the user executing the cron task.
2) If you have defined something in the "backup_sche_destination", then Moodle will store all backup files inside such directory, so verify that the path is a correct full path and that the user executing the cron task has permissions to write there.
Ciao