I have 4 cameras. I cannot view one. The other 3 load OK, but this one fails. I am logging in on the device through a cloud. I can view all the cameras on my Android app so I know that they are all working. Viewing on PC is the issue.
The first thing to try is to reset the camera completely and then reconfigure it. I know I've encountered this issue with some cameras and resetting them to factory defaults solved the problem.
Additionally, download and open the PDF file available for the NVMS application to learn how to properly add the camera, because you might be missing something that needs to be configured in order to view it.