EDIT: I finally just quit the game, unloaded Steam, rebooted the machine, restarted Steam, restarted the game, and loaded my last quick save. At that point, I could successfully exit the game and get my achievements. I have no idea what went wrong.

ORIGINAL POST: I just finished E1L3 100% (I got the chime when I hit 100%, the status bar says 100%, and when I exit the level it also says 100%). But, I can't seem to exit the level cleanly. When I go through the exit, the status at the bottom of the scoring page is a flickering "Uploading Score" that never seem to finish. My only options are Quit and Retry (to the left of "Uploading Score"). The option to Continue (I think that's what it is - anyway, it's usually to the right of the screen) is entirely missing. The problem is that Steam doesn't actually give me the 100% achievement for the level. Any idea why that "Uploading Score" is stuck there and what I should do about it?