Custom Print Problem

Hello, I'm looking for some help with printing using custom layouts.

I have published my printing service, loaded the print widget in App Builder and set it to use arcServer to print using the following URL

https://mywebserver/arcgis/rest/serv...rt%20Web%20Map

The service reads in ok and allows me to set the default format and my custom layout template.

The problem comes when I open the print widget in my flex app e.g. https://mywebserver/flexviewers/myviewer
I immediatedly get the error:

Unable to connect to GP Service:
https://mywebserver/arcgis/rest/serv...rt%20Web%20Map

GIS Server is missing a crossdomain file.
[RPC Fault faultString="Security error accessing url" faultCode="Channel.Security.Error" faultDetail="Destination: DefaultHTTPS"]

We are using a setup as follows:
Windows Domain User Store
ArcGisServer Builtin Role Store
Web Tier Authentication

Despite the error message, there is a crossdomain file at D:\ArcGIS\Server\framework\runtime\tomcat\contexts\rootapp

which reads as follows:

<?xml version="1.0"?>
-<cross-domain-policy>
<allow-access-from domain="*"/>
<site-control permitted-cross-domain-policies="all"/>
<allow-http-request-headers-from domain="*" headers="*"/>
</cross-domain-policy>

All my other services are working and accessible and the server has access rights to the folder with my custom layouts in it.
Does anyone have any ideas?

Thanks