Dear @Pierluigi,

After a OC200 configuration (Cloud OK), if I switch off a computer in the LAN network, the Cloud led in the front of a switch gone off without reason. 

Are the OC200 and the computer both connected to a Switch via Ethernet cable?

If yes, the corresponding Link/Act LED on the Switch should be OFF after you unplug the LAN port or remove power from the computer.

When you find there is a LED gone off on the Switch, please confirm whether it's an indicator for the LAN connection to the computer.

There is a Cloud LED on the OC200 itself, when it is OFF(not flashing), please confirm the OC200 is still connected properly to your network and make sure the Internet is working steadily. Then please try accessing the OC200 from the local LAN network, go to Cloud Access to verify if the Status is Connected. In addition, please check the current firmware of your OC200. Thank you.