1.) It sounds like a bad capasitor.
2.) Otherwise something in the population was mounted in reverse.
3.) It's a long shot that there's a cross in the CAD layout of the sodder due to bad CAD or a fluke in production.
More than likely it's a capasitor that melted and dripped on the PCB.
I've seen it happen dozens of times.
Anywhich way, it's a defective modem. They should swap it out for you and pay for shipping both ways with their appologies.
Power either comes from the USB port or the PS supplied by the modem manufacturer. So, they can't argue with you if it's their PS. If the modem is powered by the USB port, the modem manufacturer can't argue with you if you've plugged other devices in that USB port without any problems.
If you want a USB modem, I'd recommend a USB modem with a separate power supply rather than one that gets it's power from the USB port. I'm not sure if anyone still makes one that draws all necessary power from the USB port. We tried to make one but it wouldn't fly.