I had this wonderful error in a Flex Application that I couldn't figure out. Every time the app attempted to access the Web Service it returned the error "Security Error Accessing URL". Well, thanks to this helpful site - http://talsma.tv/post.cfm/flash-9-0-124-and-webservice-over-https-channel-security-error - my problems have been solved.
Apparently there is a security issue with Flash 9.0.124 that requires the following line to be added to the CrossDomain.xml file:
<allow-http-request-headers-from domain="*" headers="SOAPAction"/>