Audacity: Error Opening .aup file?

I created an .aup file that could open flawlessly the first few days after I finished it. Now, Audacity won't open it because it "can't find the project data folder". Well, the folder is untouched and is right in the system of my mac (I can easily locate it). Will someone please explain why Audacity cannot find the folder even if it exists and how to fix the problem?