offload: error: failed to connect to `#<input-output... Protocol error

[zimoun]

Dear,

I am not able to setup the offloading hook.

I mimmick the example:
(list
      (build-machine
        (name "my-machine.example.org")
        (system "x86_64-linux")
        (host-key "ssh-ed25519 AAAAB3Nza…")
        (user "simon")
        (private-key
         (string-append (getenv "HOME")
                        "/.ssh/identity-for-guix"))))

Then I get the error:  guix offload test

guix offload: testing 1 build machines defined in '/etc/guix/machines.scm'...
guix offload: Guix is usable on 'my-machine.example.org' (test
returned "/gnu/store/883yjkl46dxw9mzykykmbs0yzwyxm17z-test")
guix offload: 'my-machine.example.org is running GNU Guile 2.2.4
guix offload: error: failed to connect to `#<input-output: channel
(open) 2b76fc0>': Protocol error

I am not able to find what is wrong.

Moreover, if I try: guix build emacs --no-substitutes

process 4189 acquired build slot '/var/guix/offload/my-machine.example.org/0'
;;; [2019/01/21 16:58:01.231460, 0] private-key-from-file: [GSSH
ERROR] The file does not exist or permission denied:
"/root/.ssh/identity-for-guix"

I do not understand:
 1. between who is the key pairing? root or daemon or user?
 2. in which $PATH Guile needs to be in? which profile?

Thank you in advance for any insights.

All the best,
simon