Weird silverlight cross domain service call problem

So I have a Silverlight Lab that i downloaded from silverlight.net (it's part of the firestarter labs -- lab 10).

On my work machine, When I try to run it, I get the "cross-domain service call" exception. Sort of makes sense, except that I do have a ClientAccessPolicy.xml file.

Now for the weird part: If I enable Fiddler, the problem goes away, and my Silverlight app works!

Turn Fiddler off, and it fails again.

This seems to have something to do with the Proxy settings in Internet Explorer. If I disable my proxy settings, I can also get the Silverlight app to work (but I can't browse the internet, of course).

Even weirder: If I create my own Silverlight app "from scratch" and I use the same structure (separate Web Applications for the Silverlight host and the services), everything works, with no hiccups (and no need to fiddle with proxies).

Any idea what开发者_JS百科 could cause that?