Hi, as I don't like to say something wrong, I checked my octane files and found every time a scene render target...
I suspect my problem to be from using different versions of the software or before 11/28 update ( ??? ) and for my render job I needed to have both computers to be on same version.
Now it seems to be ok, except that buttons in the Node inspector like "Selects the render target node and shows it in the node inspector" works only if the Scene render target had been selected in scene outliner.
All is fine, I'm happy !