I think I've run into that problem before — or something very similar. Have you tried right-clicking on the file and using the context menu to open it? In my experience, it will then tell you that it's from an unrecognized author and are you sure you want to open it, and you tell it yes. Once it has permission to open the file, it should do so without any grief.
Not sure how exactly it happened in my situation, so I have no idea if that will work for you, but I hope it helps.