It can also just be a buggy connection at the precise moment SmartPSS calls the stream. I see it all the time when i first open SmartPSS with multiple cameras.
Simply click 1X to highlight the window, then double click on the camera in the list on the left and more often than not it will appear.
But yeah, it could be due to a network issue as I see that when my computer isnt talking to my router for whatever reason