Actually I am using proxy on both old public web application and current new ArcGIS view for Flex v 3.5. Beside that, use the same REST url. I used Fiddler to see the network traffic, I found that, crossdomain.xml being highlight with DNS Lookup Failed message. My old Flex api site didn't see crossdomain issue and able to use proxy.
Do you think I missed some setting and configuration part on ArcGIS Viewer for Flex application?
Thank you for your time.