APC Power-Saving Back-UPS Pro 550 (BR550GI)

If this was a recently purchased UPS, I would contact APC and see if they won't replace the cable. In your contact, mention that you have tried the cable on the different devices. BTW, if they won't and you want to acquire your own new cable, the end that goes into the UPS is NOT an RJ-45 plug (8 wires) but an RJ-50 (10 wires). Until a recent discussion on this forum I was also under the impression that it was an RJ-45.

Here is the discussion: http://www.readynas.com/forum/viewtopic.php?f=89&t=65926&p=367009#p367009. The post by Claykin explaining the difference is the 11th one.