It looks like AH is trying to run in a 3:4 ratio window inside your portrait view. I would double check the text inside the "video8.cfg" file in your settings folder. You may have to manually enter the resolution. Set it to your native resolution on the monitor and see if that works.
Then again, video cards don't always support all resolutions, and I've never tried this myself. I have a monitor that does this (pivots). I'll give it a try tonight and see if I can get mine working.
Will let you know what I find.