Unable to resolve resource bundle "*" for locale "en_US" (Flex, Flash Builder)

I could solve my problem by adding following Library Path to the Project settings / Flex Build Path:

C:\Program Files\Adobe\Adobe Flash Builder 4\sdks\4.0.0\frameworks\locale\en_US

It seems that the Flex compiler knows the path when working with the standard skins but somehow it can't find it when dealing with custom skins.