A number of people have been running into a handful of common configuration problems with their application, server, or the collection content when trying to use PivotViewer, so I've built a tool to help diagnose these problems. You can find it here.

Usage is like this:

  PivotViewerValidator [URL to .xap file] [URL to .cxml file]

The collection and application need to be hosted on a web server (even the Visual Studio development web server that VS starts up to run your Silverlight apps will do just fine). You'll also need at least .net 3.5 installed to run the application.

Update: The version of the tool posted earlier today seems to have been looking for some unnecessary XAP contents. I think this should be fixed now.