I manage a surveillance system myself and had the same issue while playing a video. However, I noticed that when I play a video that was recently recorded (1-2 hours) it refuses to play. If it's possible, switch the player and then try to play the videos. I know that this also happens when the software didn't embed the headers into the file, and even the built-in player doesn't know how to play them. Just wait a little longer and then play the video.
If it's possible, re-download and install the NVMS-1000 application once again as this could be a bug as well.