Custom workflow step inconsistent behavior - plugins

Question

I've made a custom workflow step which sends email to the group of users. I take precreated email and send it programmatically:
if (recepients.Entities.Any())
{
Entity email = service.Retrieve("email", mailId, new ColumnSet(true));
email.Attributes["to"] = recepients.Entities.ToArray();
service.Update(email);
SendEmailRequest sendEmail = new SendEmailRequest()
{
EmailId = mailId,
IssueSend = true
};
service.Execute(sendEmail);
}
The problem is - it works just fine in sync mode, and throw an "Invalid Cast" exception if i convert workflow into async and try to run it.
Here what trace log on the server provides:
>System.InvalidCastException: Microsoft Dynamics CRM has experienced an error. Reference number for administrators or support: #EBCA9EC1: System.InvalidCastException: Specified cast is not valid.
> at Microsoft.Crm.Common.ObjectModel.TrackingManager.GetNextTrackingToken(String subject, String& trackingToken)
> at Microsoft.Crm.Common.ObjectModel.EmailService.Send(Guid emailId, Boolean issueSend, String trackingToken, ExecutionContext context)
Somehow error occurs during converting results of the workflow. Where should I look to find the root of this problem?
at WorkflowToAsyncResultConverter.Convert(WorkflowSystemPausedResult wfResult) ilOffset = 0x23
at WorkflowToAsyncResultConverter.Convert() ilOffset = 0x9D
at WorkflowContext.EndProcessing(IGenericHandlerResult result) ilOffset = 0xC
at ActivityHost.CompleteWorkflow(IGenericHandlerResult result, WorkflowApplication activityInstance, ICommonWorkflowContext context) ilOffset = 0x70
at ActivityHostBase.OnWorkflowTerminated(WorkflowApplicationUnhandledExceptionEventArgs args, WorkflowApplication activityInstance, ICommonWorkflowContext context) ilOffset = 0x8F
at UnhandledExceptionEventHandler.OnStage1Complete(IAsyncResult lastResult, WorkflowApplication instance, Exception exception, Activity source, String sourceInstanceId) ilOffset = 0x46
at UnhandledExceptionEventHandler.Run(WorkflowApplication instance, Exception exception, Activity exceptionSource, String exceptionSourceInstanceId) ilOffset = 0x78
at WorkflowApplication.OnNotifyUnhandledException(Exception exception, Activity exceptionSource, String exceptionSourceInstanceId) ilOffset = 0x23
at ActivityExecutor.NotifyUnhandledException(Exception exception, ActivityInstance source) ilOffset = 0x18
at Scheduler.OnScheduledWork(Object state) ilOffset = 0x123
at SendOrPostThunk.UnhandledExceptionFrame(Object state) ilOffset = 0x0
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

Answer 1

Try creating an OrganizationService running as the executing user, instead of SYSTEM. A similar issue with the core email operation has been experienced by Andrii Butenko, who concluded that it works when running the service in the context of the executing user.
I have changed instantiation of service to running user and everything worked without any issues:
IOrganizationServiceFactory factory =
(IOrganizationServiceFactory)serviceProvider.GetService(typeof(IOrganizationServiceFactory));
IOrganizationService service = factory.CreateOrganizationService(executioncontext.UserId);