I get a similar message. My message is "Digital Editions could not access this device. Please make sure this device is connected to your computer. But Digital Editions lists my device under the DEVICES heading. When I disconnect the device, the listing goes away. Also, it is listed as a USB-connected device in Windows Explorer. So the device is clearly connected.
I am running Digital Editions version 4.5.6 on an up-to-date 64-bit Windows 10 PC.