I’m currently having this problem. I solved it by moving my .swf to the desktop. I think in my case, the problem is that the folder I’m using has a slash in the name. The main folder has a title that is something like “Client 1/Client 2”. The original AE project was given to me as a collected project and it worked when I opened it up from my Downloads folder, so I knew that it should be okay, but I’m going to retitle that folder as soon as my render finishes. If you’re having this problem, move things or rename your folders. I was also able to check that my .swf was still functional by dropping it into a browser window.
This is a bug. I really should be able to use any unicode character in my folder name that my OS allows.