I believe Error 9961 means the box wasn’t given an IP address.
Do you have DHCP enabled for your LAN? If not, you’ll need to allocate an IP address manually. If you do, then simply reboot your connect box, then your horizon box, and see if it fixes the issue. If not, a factory reset of your Horizon box may help.
Personally, I see no reason to connect the Horizon box to my LAN as I don’t use its DLNA (streaming) features. It doesn’t need LAN access to work as it gets its own internet connection from UPC via the same cable it uses to get the TV pictures.