After further testing, I'd be tempted to say that the issue comes from dropbox's speed of file serving.
It appears that not all images are properly cached before displaying the game.
It's not that big of a deal as long as you don't plan to only host the game in dropbox.
It's a bit strange that it happened "all of a sudden", never had this issue before a few days ago.
Just be aware that for your testing phases, it's better to wait (don't hesitate to wait a few minutes) before making any action in the game, even if the loading is supposed to be complete.
(A few refreshes seems to also help, making sure all the files are loaded)