srx archival SCP Issue

Hi, 

I'm tring to get archival configuration to work on a srx100h2.

i got an ip 10.252.0.27 on srx and 10.50.50.21 for the FTP , SFTP Server.

When i try a command like :   "file copy /var/log/messages "scp://user@10.50.50.21" " it works !! 

But when i configure the same in the srx it doesn't 

 archival {
        configuration {
            transfer-on-commit;
            archive-sites {
                "scp://user@10.50.50.21" password "XXXXXXXXXXXXXXXX"; ## SECRET-DATA
            }
        }
    }

 In log messages of the srx i got this : 

Sep 26 19:32:33  SRX logger: transfer-file failed to transfer /var/transfer/config/SRX_juniper.conf.gz_20160926_095814
Sep 26 19:32:33  SRX pfed: Error <256> uploading file '/var/transfer/config/SRX_juniper.conf.gz_20160926_095814'. Remaining retry attempts = 2, retry in 595 seconds...

In my SFTP server i see this in /var/log/auth.log 

Sep 26 19:32:32 sftp-router sshd[3008]: Accepted password for ergatel from 10.252.0.27 port 56867 ssh2
Sep 26 19:32:32 sftp-router sshd[3010]: Received disconnect from 10.252.0.27: 11: disconnected by user

 I tried everything in internet. and it does not resolve my issue. Does someone got the same or know how to resolve it ? Thanks.