cape-powered BB black works, but has no 5V voltage (and no USB)

My new BB black wireless + cape has no 5V on the USB port. I noticed it after connecting a serial-to-usb converter (no light, no /dev/ttyUSB0 etc). A voltmeter shows no 5V even on USB socket pins on the USB. Is the board decidedly malfunctioning or can it be a software issue?

I am sorry for the lack of clarity, it has no 5V only on the usb part, I guess.

The 5V is there until a device is connected. Then it disappears until the next reset.

When a low-power device is connected, like a mouse, the USB port works. If a medium-power device is connected, the port switches off until the next reset.