I finally make it worked with your help. After having updated the firmware and fixed the server path, I ran the clish command "backup automatic-mode" and it returned this error message:
Error uploading to backup, scp return code 255. Please check your premissions for the remote server.
The error message refered to the server, but the gateway hasn't even try to establish a ssh connection to the server. I proved it with a tcpdump. So I decided to switch to "TFTP" mode and it worked at the first try.
Switching back to SCP mode still returned the same error message. Can't figure what's the problem because if I use scp command from expert mode, it works.
I'll switch permanently to TFTP mode to resolve my issue.
Thanks for your help all!