I would like to share my painful experience with those who have or will encounter similar problems as I have.
I setup 4 cameras and I could not figure out why the video streams in the Montage view keep hanging, and I keep getting errors in the log.
I tried so many different suggestions over the web that I was almost ready to give up. Then I tried to lower the bitrate of the video stream and voila! The video streams are now stable!
My initial video stream settings from the cameras were 1280x720, 30fps, 4Mbps. I lowered it to 10fps, 2Mbps. Then to 5fps, 1Mbps. This was the setting that worked for me. Hope this helps someone!