Thanks Andre. But I think that the problem is clear : Nethserver’s backup module expects the target server to be smart and will not work with dumb, basic, passive, restricted, pure (S)FTP servers.
I tried other ways. SMB + Restic don’t work, because of some unexplained incompatibility.
I also tried Webdav + rync. Works more or less but performance is absolutely terrible, not usable.
Still finding my way but generally it looks like nethserver’s backup module is not really effective with dumb and restricted servers.