Oh no, that's no good! We might not have tested the game on vertical monitors >.< I'm not seeing an obvious way to change the startup resolution, but hopefully QA/engineering folks can reproduce this tomorrow and come up with a fix.
In the mean time, can you see if it's running in windowed mode or full screen? You can't change the resolution in full screen, but in windowed you should be able to. Then the confirm button might work? (Also, if you can see it, it would probably be useful to see what resolutions it's filling into the dropdown.)