Hi,
With silverlight its a very common error, firstly the error u r getting is a generic error which silverlight application shows for any service side error.
Now the solution, try the find out the root cause of this error.
There are various ways of doing this.
Below is the one which i used for mine:
1) Add the below piece of code to ur config file of the wcf service
<system.diagnostics>
<sources>
<source name="System.ServiceModel"
switchValue="Information, ActivityTracing"
propagateActivity="true">
<listeners>
<add name="traceListener"
type="System.Diagnostics.XmlWriterTraceListener"
initializeData= "D:\Sumit.svclog"/>
</listeners>
</source>
</sources>
</system.diagnostics>
this will generate an .svc, analyse this file it will provide you with a lot of information.
2) U can use external tools for analysing the wcf service calls, such as fiddler. In this case also try to analyse the service calls