Not sure if the details written to the trace log might provide any additional insight:
# CRM Tracing Version 2.0
# LocalTime: 2014-05-14 13:51:33.199
# Categories: Exception.*:Verbose
# CallStackOn: No
# ComputerName: MSRCCRM2013-DEV
# CRMVersion: 6.0.0002.0051
# DeploymentType: OnPremise
# ScaleGroup:
# ServerRole: AppServer, AsyncService, DiscoveryService, ApiServer, HelpServer, DeploymentService, SandboxServer, DeploymentManagementTools, VssWriter, EmailConnector
[2014-05-14 13:51:33.199] Process: w3wp |Organization:13e6d40d-c3cc-e311-80b8-00155d76000f |Thread: 11 |Category: Exception |User: 74d97f17-c3cc-e311-80b8-00155d76000f |Level: Error |ReqId: e7cb319c-84ef-4519-bf8f-53c679210a77 | CrmException..ctor ilOffset = 0x7
at CrmException..ctor(String message, Exception innerException, Int32 errorCode, Boolean isFlowControlException) ilOffset = 0x7
at CrmException..ctor(Exception innerException, Int32 errorCode, Object[] arguments) ilOffset = 0xB
at VersionedPluginProxyStepBase.Execute(PipelineExecutionContext context) ilOffset = 0x65
at Pipeline.Execute(PipelineExecutionContext context) ilOffset = 0x65
at MessageProcessor.Execute(PipelineExecutionContext context) ilOffset = 0x1C5
at InternalMessageDispatcher.Execute(PipelineExecutionContext context) ilOffset = 0xE4
at ExternalMessageDispatcher.ExecuteInternal(IInProcessOrganizationServiceFactory serviceFactory, IPlatformMessageDispatcherFactory dispatcherFactory, String messageName, String requestName, Int32 primaryObjectTypeCode, Int32 secondaryObjectTypeCode, ParameterCollection fields, CorrelationToken correlationToken, CallerOriginToken originToken, UserAuth userAuth, Guid callerId, Guid transactionContextId, Int32 invocationSource, Nullable`1 requestId, Version endpointVersion) ilOffset = 0x16E
at OrganizationSdkServiceInternal.ExecuteRequest(OrganizationRequest request, CorrelationToken correlationToken, CallerOriginToken callerOriginToken, WebServiceType serviceType, UserAuth userAuth, Guid targetUserId, Boolean traceRequest, OrganizationContext context, Boolean returnResponse) ilOffset = 0x16A
at OrganizationSdkServiceInternal.ExecuteRequest(OrganizationRequest request, CorrelationToken correlationToken, CallerOriginToken callerOriginToken, WebServiceType serviceType) ilOffset = 0x3D
at OrganizationSdkServiceInternal.Create(Entity entity, CorrelationToken correlationToken, CallerOriginToken callerOriginToken, WebServiceType serviceType) ilOffset = 0x37
at OrganizationSdkService.Create(Entity entity) ilOffset = 0x11
at ilOffset = 0xFFFFFFFF
at SyncMethodInvoker.Invoke(Object instance, Object[] inputs, Object[]& outputs) ilOffset = 0x241
at DispatchOperationRuntime.InvokeBegin(MessageRpc& rpc) ilOffset = 0x100
at ImmutableDispatchRuntime.ProcessMessage5(MessageRpc& rpc) ilOffset = 0x48
at ImmutableDispatchRuntime.ProcessMessage31(MessageRpc& rpc) ilOffset = 0xC6
at MessageRpc.Process(Boolean isOperationContextSet) ilOffset = 0x62
at ChannelHandler.DispatchAndReleasePump(RequestContext request, Boolean cleanThread, OperationContext currentOperationContext) ilOffset = 0x256
at ChannelHandler.HandleRequest(RequestContext request, OperationContext currentOperationContext) ilOffset = 0xF1
at ChannelHandler.AsyncMessagePump(IAsyncResult result) ilOffset = 0x39
at AsyncThunk.UnhandledExceptionFrame(IAsyncResult result) ilOffset = 0x0
at AsyncResult.Complete(Boolean completedSynchronously) ilOffset = 0xC2
at ReceiveItemAndVerifySecurityAsyncResult`2.InnerTryReceiveCompletedCallback(IAsyncResult result) ilOffset = 0x58
at AsyncThunk.UnhandledExceptionFrame(IAsyncResult result) ilOffset = 0x0
at AsyncResult.Complete(Boolean completedSynchronously) ilOffset = 0xC2
at AsyncQueueReader.Set(Item item) ilOffset = 0x21
at InputQueue`1.Dispatch() ilOffset = 0x121
at ScheduledOverlapped.IOCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* nativeOverlapped) ilOffset = 0x22
at IOCompletionThunk.UnhandledExceptionFrame(UInt32 error, UInt32 bytesRead, NativeOverlapped* nativeOverlapped) ilOffset = 0x5
at _IOCompletionCallback.PerformIOCompletionCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* pOVERLAP) ilOffset = 0x3C
>Crm Exception: Message: An unexpected error occurred., ErrorCode: -2147220970, InnerException: System.NullReferenceException: Object reference not set to an instance of an object.
at Microsoft.Crm.BusinessEntities.SecurityAttributes..ctor(SecurityTraits traits, BusinessEntity entity, ExecutionContext context)
at Microsoft.Crm.ObjectModel.SystemUserServiceInternal`1.Create(IBusinessEntity systemuser, ExecutionContext context)