that's a security issue triggered by trying to use the bitmapdata of images loaded across a security sandbox.
you need cross-domain policy files.
Thanks for the reponse, but did you read this part of my post?:
"the security setup has also been done correctly (the image server has the crossdomain.xml file, and the local SWF file has the relevant Security.allowDomain settings in place)."
As specified, this was working pre the 10.1 update, and is not working now. No code has changed in the file for 2 years now, and this problem never exhibited itself before.
Any help would be appreciated.
again, you need cross-domain policy files.
you need a meta-policy file in the root of the data-supplying server and (unless your meta-policy file also allows your domain to access everything on the server) you'll need a policy file.