Error by booting with microSD/USB

[ItachiSan]

Hello guys,
I followed the official guide but I've problems booting the board and I've the problem described in the topic title.
Actually, the 4 USR leds stands still if I connect the board with a USB cable to my computer.
It happens even sometimes (I don't get if it's random or not) if I plug my board to a common charger through the USB cable.