I'm not sure if you're still facing that issue - but I've resolved mine. I have a Lupus 500 UPS which is displayed as "Fry Electronics".
Try to download NUT by pasting this link and choose blazer_usb driver. It should come up at the bottom of the page.
FYI - I'm on unRAID 6.3.4.
FYI 2 -Tested with power outage, array is first being stopped and then system is shut down.