I was able to get this problem fixed for me with the tech support help and would like to chime in on what we did..
As in my OP, I couldn't get the screen to fill the screen and even with 'Fullscreen stretch", it still didn't work, I just had to uncheck the "constrain proportions" box and the black borders on the side went away. Hope this helps you in this situation also.
I'm using a 1 operator subscription and I love the program, the only thing that I can't figure out is when I connect to any host, the viewer has two large back bars on either side of the screen. It's like the viewer cannot fill up the screen? I tried F12 and going through the different sizes, normal, stretch, full, full stretch and they all have the black bar on each side. I have tried adjusting the resolution (1366x768) on my viewer screen to a lower resolution (1024x768) but that doesn't help.