Try this 'double luck' workaround of the C-8 pairing issue :
- Bring the problematic Zigbee 3.0 device very close to your C-8 hub.
- Go to Settings->Zigbee Details and click the 'Rebuild Network' button.
- Wait for the process to finish (the button will become enabled again)
- Click the same 'Rebuild Network' button again and wait for the process to finish.
- Go to Devices -> Add Device -> Zigbee and click on the 'Pair while trying to avoid Zigbee 3.0 repeaters ' link, confirm.
- Start the Zigbee pairing on the device (press the pairing button for 4-5 seconds).
Please follow the steps 1..5 precisely, not skipping and not adding anything else.