[CentOS] crontab won't work

Hi,

I have a strange problem here. I have two servers, one dedicated 
webserver, and a local backup server.

On the webserver I have this :

[root at 12569hpv163154:~] # crontab -l
00 22 * * * /usr/local/bin/sauvegarde-pmbccps.sh

This launches a backup script every day at 22:00, and it works as expected.

Now here's what I have on the local backup server :

[root at grossebertha:~] # crontab -l
24 17 * * * /usr/local/bin/sauvegarde.sh

Here, the script /usr/local/bin/sauvegarde.sh basically launches an 
incremental backup with rsync over SSH. The script is executable OK, it 
works very well when launched by hand on the command line... except cron 
won't start it. Here for example I tried to launch it on 17:24. Nothing. 
I checked the machine 'date', it's OK.

Now I don't have the slightest clue why this doesn't work. What's the 
magic word here ?

Cheers,

Niki