A firmware update was recently released for the NVR6XX-4KS2 series devices.
Information, instructions, and download links can be found Here
Troubleshoot/NVR/Video Loss 1 Camera
From Dahua Wiki
Video Loss 1 Camera
There are following possibilities:
- Firmware is not compatible. You will need to upgrade to the latest version.
- Brightness is 0. Please restore factory default setup.
- There is no video input signal or it is too weak.
- You will need to check the network cable.
- DVR hardware malfunctions.