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.
Screen not full screen..still has black borders on left and right side of screen..
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.