The error 678 is related to the PPPoA Protocol which is being used by your modem and occurs due to the type of connection type used : Ethernet instead of USB (the wrong driver is being used).
- Check if the appropriate drivers are installed and being used : Start => Control Panel => Network Connections
- Then right-click on Connection => Properties.
- Go to the General tab and check the if the right driver is being used (must correspond to the modem's brand and series).
- Re-install driver if necessary.
Original document published by ninha25.