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 All Cameras
From Dahua Wiki
Video Loss on Multiple Cameras
There are following possibilities:
- Firmware may be old. Please upgrade to the latest version.
- Brightness is 0. Please restore factory default setup.
- If There is no video input signal or it is too weak, due to the length of cable, find shorter cable.
- Check privacy mask setup or your screen saver and remove them if possible.
- If DVR hardware malfunctions then change hardware.
- If Not enough bandwidth then check internet.
- If Insufficient power then check and change power supply.
- Check if you are using You are using aluminum braid or copper braid cable and change if not.