I would try one factory reset after you do a Save Configuration to File. See if the factory reset does anything. I'm not sure it will. If your exhibiting the same issue on different PCs with the latest SP SW, and the printer works when directly connected to a PC using it's own drivers, I would presume there could be a compatibility issue with the printer.
One thing you can do, do a factory reset with the printer connected to the USB port. If SP doesn't see it. Power OFF the router then back ON with out the USB connected. Let the router come to ready. Then plug in the USB cable. After you do this, look a the routers logs, you should see some USB connection entries here. If you don't then I presume that this needs D-Link support, level 3 or higher. I have seen some issues with various printers not working with SP on the USB. If you phone contact them, give the details and what you have done already. I hope there might be a solution.
If there isn't a solution to the SP and USB for the printer, as a suggested alternative in using the USB and Shareport, get you a USB to LAN print server device. This will take the USB and make a connection to an LAN IP address for any PC or device to connect to the printer and use. D-Link has a couple of print server devices out there that should work. If you get on the phone, ask them as well about this as an alternative and what product they would recommend.
Keep us posted on how it goes.