Hello brubble1,
Do you see this same entry in the log for your latest attempts?
10/06/15 15:06:01 WARN Host: 10.10.156.52;ESN: 000456C93E82;Message: Sending Upgrade command with URL: http://fe80:0:0:0:f583:3e60:6b8f:2535%10:80/1579512806/ePMP-NonGPS_Synced-v2.5.1.tar.gz (SITE= CambiumNetworks)
If you do, could you try disabling IPv6 from the properties of the NIC you are using with CNUT?
Regards