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I am experiencing precisely this problem (during the final hours of the Global Game Jam)! However, in my case THE PROJECT WILL NOT OPEN AT ALL!...
This appears to be a recurring problem, with many users (Windows and Mac) but NO ONE can point to a definitive cause/solution.
I have also just downloaded Unity 5 and my excitement was instantly dispelled when I faced this error dialog. There seems to be no choice but "Quit".