The response received from the service didn't contain valid XML. ---> System.Xml.XmlException: Root element is missing

my process has been running on server for a while. Last night, i got an exception. the error only occurred once so far. At next run time, the BOT works properly. The full exception description as the following:
An Exception Occurred at 03/30/2020 22:45:45 - Exception has been thrown by the target of an invocation. and InnerException: Microsoft.Exchange.WebServices.Data.ServiceRequestException: The response received from the service didn’t contain valid XML. —> System.Xml.XmlException: Root element is missing.
at System.Xml.XmlTextReaderImpl.Throw(Exception e)
at System.Xml.XmlTextReaderImpl.ParseDocumentContent()
at System.Xml.XmlTextReaderImpl.Read()
at System.Xml.XmlTextReader.Read()
at System.Xml.XmlCharCheckingReader.Read()
at Microsoft.Exchange.WebServices.Data.EwsXmlReader.Read() in \REDMOND\EXCHANGE\BUILD\E15\15.00.0913.015\SOURCES\sources\dev\EwsManagedApi\src\EwsManagedApi\Core\EwsXmlReader.cs:line 157
at Microsoft.Exchange.WebServices.Data.EwsXmlReader.Read(XmlNodeType nodeType) in \REDMOND\EXCHANGE\BUILD\E15\15.00.0913.015\SOURCES\sources\dev\EwsManagedApi\src\EwsManagedApi\Core\EwsXmlReader.cs:line 170
at Microsoft.Exchange.WebServices.Data.ServiceRequestBase.ReadXmlDeclaration(EwsServiceXmlReader reader) in \REDMOND\EXCHANGE\BUILD\E15\15.00.0913.015\SOURCES\sources\dev\EwsManagedApi\src\EwsManagedApi\Core\Requests\ServiceRequestBase.cs:line 1083
— End of inner exception stack trace —
at Microsoft.Exchange.WebServices.Data.ServiceRequestBase.ReadXmlDeclaration(EwsServiceXmlReader reader) in \REDMOND\EXCHANGE\BUILD\E15\15.00.0913.015\SOURCES\sources\dev\EwsManagedApi\src\EwsManagedApi\Core\Requests\ServiceRequestBase.cs:line 1087
at Microsoft.Exchange.WebServices.Data.ServiceRequestBase.ReadSoapFault(EwsServiceXmlReader reader) in \REDMOND\EXCHANGE\BUILD\E15\15.00.0913.015\SOURCES\sources\dev\EwsManagedApi\src\EwsManagedApi\Core\Requests\ServiceRequestBase.cs:line 591
at Microsoft.Exchange.WebServices.Data.ServiceRequestBase.ProcessWebException(WebException webException) in \REDMOND\EXCHANGE\BUILD\E15\15.00.0913.015\SOURCES\sources\dev\EwsManagedApi\src\EwsManagedApi\Core\Requests\ServiceRequestBase.cs:line 934
at Microsoft.Exchange.WebServices.Data.ServiceRequestBase.GetEwsHttpWebResponse(IEwsHttpWebRequest request) in \REDMOND\EXCHANGE\BUILD\E15\15.00.0913.015\SOURCES\sources\dev\EwsManagedApi\src\EwsManagedApi\Core\Requests\ServiceRequestBase.cs:line 827
at Microsoft.Exchange.WebServices.Data.ServiceRequestBase.ValidateAndEmitRequest(IEwsHttpWebRequest& request) in \REDMOND\EXCHANGE\BUILD\E15\15.00.0913.015\SOURCES\sources\dev\EwsManagedApi\src\EwsManagedApi\Core\Requests\ServiceRequestBase.cs:line 724
at Microsoft.Exchange.WebServices.Data.MultiResponseServiceRequest1.Execute() in \\REDMOND\EXCHANGE\BUILD\E15\15.00.0913.015\SOURCES\sources\dev\EwsManagedApi\src\EwsManagedApi\Core\Requests\MultiResponseServiceRequest.cs:line 157 at Microsoft.Exchange.WebServices.Data.ExchangeService.InternalCreateItems(IEnumerable1 items, FolderId parentFolderId, Nullable1 messageDisposition, Nullable1 sendInvitationsMode, ServiceErrorHandling errorHandling) in \REDMOND\EXCHANGE\BUILD\E15\15.00.0913.015\SOURCES\sources\dev\EwsManagedApi\src\EwsManagedApi\Core\ExchangeService.cs:line 401
at Microsoft.Exchange.WebServices.Data.Item.InternalCreate(FolderId parentFolderId, Nullable1 messageDisposition, Nullable1 sendInvitationsMode) in \REDMOND\EXCHANGE\BUILD\E15\15.00.0913.015\SOURCES\sources\dev\EwsManagedApi\src\EwsManagedApi\Core\ServiceObjects\Items\Item.cs:line 198
at Microsoft.Exchange.WebServices.Data.EmailMessage.InternalSend(FolderId parentFolderId, MessageDisposition messageDisposition) in \REDMOND\EXCHANGE\BUILD\E15\15.00.0913.015\SOURCES\sources\dev\EwsManagedApi\src\EwsManagedApi\Core\ServiceObjects\Items\EmailMessage.cs:line 110
at UiPathCodeRunner_dc0e4749912e4e2d8e36bf135050ee08.Run(arguments including username and password); source - mscorlib; StackTrace - at System.RuntimeMethodHandle.InvokeMethod(Object target, Object arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object parameters, Object arguments)
at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object parameters, CultureInfo culture)
at System.RuntimeType.InvokeMember(String name, BindingFlags bindingFlags, Binder binder, Object target, Object providedArgs, ParameterModifier modifiers, CultureInfo culture, String namedParams)
at UiPath.Core.Activities.Workflow.CompilerRunner.Run(Object args)
at UiPath.Core.Activities.InvokeCode.Execute(CodeActivityContext context)
at System.Activities.CodeActivity.InternalExecute(ActivityInstance instance, ActivityExecutor executor, BookmarkManager bookmarkManager)
at System.Activities.ActivityInstance.Execute(ActivityExecutor executor, BookmarkManager bookmarkManager)
at System.Activities.Runtime.ActivityExecutor.ExecuteActivityWorkItem.ExecuteBody(ActivityExecutor executor, BookmarkManager bookmarkManager, Location resultLocation); TargetSite - System.Object InvokeMethod(System.Object, System.Object, System.Signature, Boolean)

is there any one knows what happen?

thanks in advance,