Calling webservice problem

Hi,

We developed a rest WS which we will call from an external application.

The ws is getting an account based on same search criteria.

when I launch the WS from visual studio (IIS express) all work fine, i get the result and i have no problem.

but once my WS deployed on the server's IIS , it does not work :the line of code that is returning the error is the retrieve.

the error I'am getting 

ERROR AccountController FindAndGetResponse - System.ServiceModel.FaultException: Le serveur n'a pas pu traiter la demande en raison d'une erreur interne. Pour plus d'informations sur l'erreur, activez IncludeExceptionDetailInFaults (depuis ServiceBehaviorAttribute ou depuis le comportement de configuration <serviceDebug>) sur le serveur pour renvoyer les informations de l'exception au client, ou activez le suivi conformément à la documentation du Kit de développement logiciel (SDK) de Microsoft .NET Framework et examinez les journaux de suivi du serveur.

Server stack trace: 
   à System.ServiceModel.Channels.ServiceChannel.ThrowIfFaultUnderstood(Message reply, MessageFault fault, String action, MessageVersion version, FaultConverter faultConverter)à System.ServiceModel.Channels.ServiceChannel.HandleReply(ProxyOperationRuntime operation, ProxyRpc& rpc)à System.ServiceModel.Channels.ServiceChannel.Call(String action, Boolean oneway, ProxyOperationRuntime operation, Object[] ins, Object[] outs, TimeSpan timeout)à System.ServiceModel.Channels.ServiceChannelProxy.InvokeService(IMethodCallMessage methodCall, ProxyOperationRuntime operation)à System.ServiceModel.Channels.ServiceChannelProxy.Invoke(IMessage message)

Exception rethrown at [0]: 
   à System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg)à System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type)à Microsoft.Xrm.Sdk.IOrganizationService.RetrieveMultiple(QueryBase query)à Microsoft.Xrm.Sdk.Client.OrganizationServiceProxy.RetrieveMultipleCore(QueryBase query)