Here is an answer I got from APC when one of my client got the same problem as you did:
The error message would pop up if the link between the computer and the UPS is weak. I cannot rule out a problem with the cable or the UPS. In order to confirm the same please try the following steps which might help :
1) Ensure that the USB cable is properly connected to the USB port of the computer.
2) If the PC was powered on before the UPS, communication will not be achieved until you disconnect and reconnect the USB cable to the host. In this case, you need to disconnect and reconnect the USB cable to establish communication.
3) Some other software is polling your USB port for another device, thus causing the UPS software to temporarily lose communication with your UPS. If this is the case, you would need to shut down that application.
4) If you have any other USB devices connected to the PC, disconnect them and then try plugging in the cable.
If the issue persists, please visit the following link which lists few troubleshooting steps :http://nam-en.apc.com/cgi-bin/nam_en.cfg/php/enduser/std_adp.php?p_sid=oXhRFGEh&p_lva=&p_faqid=2923