I have the same issue, multiplayer fails to load about 50% of the time. It *seems* to help if I completely quit the game, close all windows, and then start from a fresh load of the client, but this could just be my imagination.
Of course, you can't use the bug reporting feature of the client at this point because it seems to load behind the blank screen (but waving your mouse around seems to show that the window is there and working, you just can't see it).
-E