is probably due to your having mucked up the
workspace\[PROJECT_NAME]\.actionScriptPropertiesfile. In my case, a libraryPathEntry had gone missing. (Google has yet to learn this trick, which would have saved a piece of my Friday.)
In short, restore from an Eclipse project backup or regenerate a new project.
I have never experienced this problem (yet!!) but my guess is that your workspace paths are "mucked up".
Have you changed any drives/paths? I notice you are not working in the C drive...