The server was unable to process the request due to an internal error

 The server was unable to process the request due to an internal error

To turn on includeExceptionDetailInFaults, add the following tag in the web.config:
<system.serviceModel>
<behaviors>
            <serviceBehaviors>
                <behavior name="">
                    <serviceMetadata httpGetEnabled="true" />
                    <serviceDebug includeExceptionDetailInFaults="true" />
                </behavior>
            </serviceBehaviors>
        </behaviors>


...
</system.serviceModel>