Channel Security error on MapManager

I run into this error message on MapManager for ArcGIS View for Flex web application v3.5 when try to open web application to public to use. I tested this on internal web browser and it works fine only happen if public try to access the site.
Here is the error message I got:

MyLayer layer failed to load: Fault code:
Channel.Security.Error

Fault.info:Security error accessing url

Fault details: Destination: Default:HTTP

I have crossdomain.xml on C:\inetpub\wwwroot folder

<?xml version="1.0"?>
<!DOCTYPE cross-domain-policy SYSTEM "http://www.adobe.com/xml/dtds/cross-domain-policy.dtd">
<cross-domain-policy>
<site-control permitted-cross-domain-policies="all"/>
<allow-access-from domain="*" secure="false"/>
<allow-http-request-headers-from domain="*" headers="*" secure="false"/>
</cross-domain-policy>

Besides that, I have another custom flex web application open to public using the same secure map services, proxy, and crossdomain without any error or problem at all. Only on ArcGIS Viewer for Flex web application got error message.
Have any ideas for this error message? Thanks.