As i said before, Uncle Frank put me on the good way (DHCP issue). I tried before many options on the switch, and no one worked. My method for this was really empiric : many tests, many failed tests and - at least with DHCP relay - i succeed. It’s strange that a DHCP relay is necessary : all pc’s are on the same subnet. I don’t uderstand why, and i’m a little bit annoyed about this but it works for now. I made a upload/download with success. I hope that this DHCP relay won’t generate some issues on the network !