Getting "Security sandbox violation" when loading PHP files from SAME domain

I’m running a webgame with a bunch of users. Recently, some have complained about the game crashing on startup. Using a debugger, I found the error:

Error #2044: Unhandled securityError:. text=Error #2048: Security sandbox violation: http://www.mydomain.com/game/game10.swf cannot load data from Error #2032: Stream Error. URL: http://www.mydomain.com/game/php/loadStartData.php

Now, this error only occurs for some user accounts. Others are able to play just fine on the same computer, but logged in with a different account. In other words, it’s not related to browser, Flash Player version, etc. Most users load the game fine, though.

I’ve searched around on the web, and the error seems to be common when you try to load resources between domains, in which case a crossdomain file is the answer. However, here, I’m just trying to load a PHP file in a subfolder (“game”) which is in folder where the swf file is residing.

What’s even weirder is that other PHP files in the same subfolder will load fine using the exact same code. And again, this is all just for some users. Other users run the game fine on the same computer.

The AS3 code that tries to load the PHP file is simply

var startDataPath:String = (“php/loadStartData.php”);
var startDataPHPRequest:URLRequest = new URLRequest(startDataPath);
var startDataPHPLoader:URLLoader = new URLLoader();
startDataPHPLoader.addEventListener(Event.COMPLETE, sortStartDataXML);
startDataPHPLoader.load(startDataPHPRequest);

Anyone have any experience with this?