This command: ./backup.sh tells me that the backup.sh script is not to be found in the PATH variable. Cron (which one are you actually using?) has an even more limited PATH (if any).
0 23 * * 0,1,2,3,4,5 /backup.sh
0 23 * * 0,1,2,3,4,5 cd /path/to/program ; ./backup.sh
0 23 * * 0,1,2,3,4,5 /full/path/to/backup.sh
Hope this helps.