Thanks for the advice.
I proceeded with new tests. Actually, it seems that the malfunction comes from one of my device, my dishwasher…
I did fully reset the XGW3000 gateway, paired it with only one device (washing machine) and it worked.
2nd device (dryer) added : still ok.
It’s only when I added the third one that the plugin got black. On the contrary, the iPhone apps works correctly, as well as the Gateway web mini-browser… This dishwasher is rather old (G2730Sci : one of the first [email protected] 1.0 compliant apliances) and is not officially supposed to work with the [email protected] 2.0 system. Pairing it with the gateway was by the way a real pain. I had to start the pairing process several times and the only way I made it succeed was by putting the gateway just close to the device and using the button on the XKM3000 module to start the connection process, instead of the menu on the user interface.
My intent now is to try to see if a firmware upgrade of this appliance is possible, through the Miele technical after-sale service… If so, it may solve the issue.