I've migrated my project from flex 3 to flex 4 and all it's ok, the new project work fine.
But I created my custom TitleWindow MX style and I created a swc library from Flash builder 4, all it's ok.
When I use my custom TitleWindow in new project (compatible with mx+spark) it work fine when I add only mx component, when I add a <s:TextArea> for example, it throw this error in run-time: Skin for ForumTitleWindow1220.boxMessaggioInput.TextArea1227 cannot be found.
where ForumTitleWindow1220 and boxMessaggioInput are my instance.
in particolar the error is "throw(new Error(resourceManager.getString("components", "skinNotFound", [this])));"
in SkinnableComponent to line 698.