What happens if you take of the file:// and just use “D:...”?
Unfortunately the prefix "file:///" presence/absence doesn't matter. Anyway I get the error.
Eventually, I found out the reason of the error. It seems due to security reasons pathes including ".." (up directory) are considered as unsafe!
Unfortunately, it's not mentioned explicitly that the usage of such notation causes the security error.
The solution is to process pathes before pass them to URLRequest e.g. by using the following routine:
private function canonicalize_path(fn: String): String
var f: File = new File(fn);