Error while adding a task to project (CSOM)

Hi everyone,

I am working on a SP Online (PWA) addin and I am trying to create a task in a project with the following code:

        public void AddTask( Guid projectUid )
        {
            using ( ProjectContext projectContext = spAccessManager.CreateUserProjectContextForSPHost() )
            {
                var pubProject = projectContext.Projects.GetByGuid( projectUid );
                var currentUser = projectContext.Web.CurrentUser;

                projectContext.Load( pubProject );
                projectContext.Load( currentUser );
                projectContext.ExecuteQuery();

                // Always checkin
                if ( pubProject.IsCheckedOut )
                {
                    // Assuming the project is checked out to the current user
                    projectContext.WaitForQueue( pubProject.Draft.CheckIn( false ), int.MaxValue );
                }

                // Load tasks
                var draftProject = pubProject.CheckOut();
                projectContext.Load( draftProject );
                projectContext.Load( draftProject.Tasks);
                projectContext.ExecuteQuery();

                // Create new task
                var taskInfo = new TaskCreationInformation();
                taskInfo.Id = Guid.NewGuid();
                taskInfo.Name = "_" + DateTime.Now;
                taskInfo.IsManual = false;
                taskInfo.Duration = "1d";
                draftProject.Tasks.Add( taskInfo );

                var job = draftProject.Update();
                // result is Success...
                var result = projectContext.WaitForQueue( job, int.MaxValue );

                // but this throws an exception because there is an error from the queueu from last operation
                job = draftProject.Publish( false );
                var result2 = projectContext.WaitForQueue( job, int.MaxValue );
            }
        }

"spAccessManager" is a custom object generating the ProjectContext and it is using the default functionality in TokenHelper.cs and SharepointContext.cs from the VS template.

I use this code just as a proof of concept and I intentionally checkin and checkout every time. In theory it should work, but when the project is checked out in the browser - an error occurs after the Update operation (despite the fact that the result is Success):

• Project:
  • ProjectSchedulingEngineException (9133). Details: id='9133'
    name='ProjectSchedulingEngineException'
    uid='7e0bcea7-30e1-e511-80c8-00155d58bd1e'
    exception='System.NullReferenceException: Object reference not set to an instance of an object. at Microsoft.Office.Project.Server.BusinessLayer.ProjectEngineUpdate.UpdateProjectWithPCS(IPlatformContext context, ProjectDataSet dataset, Guid projectUid, Guid sessionUid, Boolean inVisibilitySyncMode, Boolean skipEventHandlers, Boolean skipPostSSPUpdate) at Microsoft.Office.Project.Server.BusinessLayer.ProjectMain.HandleProject2(MessageContext mContext, ProjectDataSet dataset, Guid projectUid, Guid sessionUid, Boolean inVisibilitySyncMode)'
    .
• Queue:
  • GeneralQueueJobFailed (26000) - ProjectUpdate.ProjectUpdate2Message. Details: id='26000'
    name='GeneralQueueJobFailed'
    uid='7f0bcea7-30e1-e511-80c8-00155d58bd1e'
    ....................

If I close the project in the browser (from the button in the ribbon) without checking it in, the code executes successfully. Also when I wait for about 20 minutes, the code is running again as expected.

This is a pretty simple operation, I did not expect such issues. Has anyone had similiar problem? Any ideas what might be causing this?