A Silverlight webservice is returning an exception, so what happened to that webservice, if already deployed you have 2 options, configure your machine to run the code on your development computer, where you never see the problem as always works in my computer, or enable that server to return the exception message on the body for you to at least see.

<faultcode xmlns:a="http://schemas.microsoft.com/net/2005/12/windowscommunicationfoundation/dispatcher">a:InternalServiceFault</faultcode><faultstring xml:lang="en-US">The server was unable to process the request due to an internal error.  For more information about the error, either turn on IncludeExceptionDetailInFaults (either from ServiceBehaviorAttribute or from the &lt;serviceDebug&gt; configuration behavior) on the server in order to send the exception information back to the client, or turn on tracing as per the Microsoft .NET Framework 3.0 SDK documentation and inspect the server trace logs.</faultstring></s:Fault>

If you haven’t install Fiddle that will give you the communication messages between your Silverlight application and the Service, check this is true on the service.

<behavior name="ServiceBehaviorConfiguration">
       <serviceDebug includeExceptionDetailInFaults="true" />
     </behavior>

Now the service will send you the exception as a reply on the service for you to see when using Fiddle.

Hope this helps

Cheers

Al