SCP/RSYNC don't work in Crontab but work when run standalone

I need to copy a backup file to a backup server everyday. I am using PublicKey SSH authentication.

All of the things work but not together:

If I run this line in crontab

rsync /home/ubuntu/*date_backup.archive* ssh centos@10.x.x.x/home/centos/backup

This works and sends the backup correctly

But since the date in the filename of the .archive changes, I need to run a script. Then I create rsync_backup.sh

#!/bin/bash
date= ...
rsync /home/ubuntu/*date_backup.archive* ssh centos@10.x.x.x/home/centos/backup

Then, if I run this via terminal with bash rsync_backup.sh, it works and sends the backup correctly

BUT if I put

* * * * * bash /home/ubuntu/rsync_backup.sh

or

* * * * * bash /home/ubuntu/rsync_backup.sh

The backup doesn't reach the destination host.

Also, if I put in the script another test line on local (mkdir /home/centos/test) and launch with crontab, it works!

I tried also with SCP, still the same results!

Do you have some ideas? Source machine is Ubuntu and Destination is Centos

Many thanks in advance

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