Loading policy files causes a deadlock when the code is called from external interface
Since policy files are immediately necessary they are downloading in a blocking manner. Since the download happens using browser provided functionality it will deadlock when invoked from code called as an ExternalInterface callback, since the browser is waiting for an answer and won't deliver the requested file.
A possible solution would be to use internal libcurl to download policy files (the problem only happens for them since they are currently downloaded synchronously). The other possibility is to make policy files download asynchronous, which is seems to be possible by a quick look at the ActionScript spec.