Keys error with Rsync SSH Task

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I'm doing the Rsync SSH test using two TrueNAS SCALE 22.12.3.3.
From BCK1 (192.168.51.180) to BCK2 (192.168.51.181).
On BCK1

1. I created a SSH key pairs
2. in the john user tab (with "admin" rights) I loaded the public key
3. I created a connection on BCK2 using the admin user and john, loading the key I generated before
4. I created an RSync task, Mode SSH, using the user john; in its Home Dir there is .ssh/authorized_keys .

I checked the documentation several times and redid the procedure several times, but when I save, the error continues to appear under the username:
In order to use rsync over SSH you need a user with a private key (DSA/ECDSA/RSA) set up in home dir.

I don't understand where the mistake is.
This post talks about ID_RSA, but I don't understand what it refers to.
https://www.truenas.com/community/threads/rsync-task-over-ssh-privat-key-must-be-named-id_rsa-how-to-use-other-named.92755/

I tried renaming authorized_keys to id_rsa, now it accepts it but with the error
KeyImportError Invalid private key

I also copied this file to the homedir of user john on Bck2.
Same error.

Obviously the files have 600 rights.

I'm also having trouble because the documentation almost never refers to the menus and panels on this version of TrueNAS.

=== Update ===
For my needs, now in the test I used RSync Module, with success, but I would like to understand why the one with SSH shouldn't work.

The two NAS are VMs that I created just for testing.
BCK2 is a clone of BCK1 and therefore general configuration and users are the same.
All other configurations are obviously different, but I followed the various pages of documentation.