ArgumentError: Error #1063: Argument count mismatch on mx.core::CrossDomainRSLItem(). Expected 5, got 7.
at mx.managers::SystemManager/http://www.adobe.com/2006/flex/mx/internal::initialize()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\managers\SystemManager.as:1878]
at mx.managers::SystemManager/initHandler()[E:\dev\4.0.0\frameworks\projects\framework\src\mx\managers\SystemManager.as:2419]

This was an easy one to fix. Your main app is compiling with a different Flex SDK than one of your modules (RSL). Check each module Flex Project Compiler properties and make sure they're all on the same sdk.