Just to be sure, I launched a project (I'm using FlashDevelop for testing) and said CEV3-1-9-Starling-Feathers-Box2D.swc as the only linked library . The project compiles fine with no errors (I'm using StarlingCitrusEngine of course) . swf version is 21 and flex4.6/Air3.9. I don't know how we could help with this.
And the fact that you see Aymeric's path is not an issue, all that matters in those is the relative path to "which file caused the error".
Are you able to try on a fresh/clean project with your IDE (in that case you just need an empty document class extending StarlingCitrusEngine to be sure I guess.)