I have never trusted dropbox and I have to use it everyday. I have seen problems with saving and sometimes it creates "conflicted files" when saving due to having 5 computers linked to the same dropbox.
As far as Scott's error message, generally, if you close Chief and Windows Explorer (if open), and then try opening again, it will. I sometimes think it's a Windows thing with that error message. Oh, When closing a file from with-in the dropbox, be sure the dropbox has finished updating. You'll see the dropbox icon spinning when it's updating.