Resolutions of 800x600 aren't supported with our client, so it's not really a surprise that things are missing there. The rest of the GUI is probably drawn outside of the screen where you can't see them. Even if they weren't, they wouldn't properly fit in the 800 width space that is available. You need at least 1024.
What is surprising though is that you see even fewer things when you use a higher resolution. What's the maximum your monitor can support? Can you take a screenshot of what that looks like in game?
2048x1536 is my maximum resolution, and switching to it technically solved the problem (I could see all the elements that should be there, including the message box), but try and play on a 17'' monitor set to 2048x1536 - as you could imagine, it's not really possible (everything is so small!).
It prompted me to try a number of different resolutions though, and to experiment with other options, and it seems that in order to get the game to display properly, I need to use a resolution of no less than 1024x768 *and* to have fullscreen enabled at the same time.
However, I still have the issue with bugged edge scrolling. If I drag the cursor to the edge of the screen, it usually has no effect, so I need to use arrows instead which is a pain in the ass. Curiously, I have no such problem in 800x600. If you can think of any possible solutions, please let me know.