Isn't it great when the answers are out there? I finally got a Google hint and overcame this one. So here are a few reasons that I encountered and overcame this error. We can easily reproduce (and often fix) this error. This occurs on the asynchronous return from a Silverlight 4 to RIA services. It's as if for any of the few reasons it fails, the call just gets aborted and we get a head-scratching "Not Found". We can sometimes even hit a debugger breakpoint in the Domain Service that gets the IQueryable call and wonder why the server appears to work in the debugger.
Here are a few suggestions to help.
Returning too much data for the object graph. Solution: Edit the 'maxItemsInObjectGraph' setting in the web.config.
Throwing an unhandled error on the service logic. It can't get back to the client. Solution: Comment out logic and see if the call succeeds.
Solution: Only enable Anonymous authentication. The request can then avoid authentication errors and possibly return successfully. This one got me again tonight when deployed a new client’s project for the first time. It also confused several of us at the Dallas Silverlight and WP7 DevCamp that I mentored at the other day. The project will work great in the local development server but when deployed to IIS, we get a Not Found error. So try disabling the Forms and Basic Authentication with only Anonymous enabled. I'm convinced that this is the most likely reason for most people.
Hope this helps others with this unintuitive error message....Hopefully it's "Now Found".