Error #1034: Type Coercion failed: cannot convert bla to IFlexModule
Finally the problem was... the CSS filename (bla.css), was the same as the application MXML file (bla.mxml)! Changing the name of the CSS file to something different (app.css) basically solved the problem. Guess when you compile a CSS file to SWF, internally the generated ActionScript class is named the same as your CSS file and thus conflicts with the main application classname.
To learn how to load at runtime, read the "Loading style sheets at run time" in the Flex Livedocs.