[+++crowbar+++]

You'll get a 404 error, when the object does not exist. Heres an excerpt of the mutator code, that makes the check.

function HTTPError(int ErrorCode)
{
	if(ErrorCode == 404)
	{
		log("##### UTSTATS-Trigger: 404 ERROR");
	}
	 else
		Failure();
}

Your ini file is okay. Even if the key is wrong, there should not be a 404 error. I don't have any idea, why the mutator does not work for you. Maybe your UT server is unable to send the HTTP request. Firewall?