Project Online Workflow Error

Question

RequestorId: 20b4b32a-9337-99eb-0000-000000000000. Details: An unhandled exception occurred during the execution of the workflow instance. Exception details: System.ApplicationException: HTTP 500 {"Transfer-Encoding":["chunked"],"X-SharePointHealthScore":["0"],"X-SP-SERVERSTATE":["ReadOnly=0"],"DATASERVICEVERSION":["3.0"],"SPClientServiceRequestDuration":["496"],"SPRequestDuration":["718"],"SPRequestGuid":["20b4b32a-9337-99eb-81e9-84ae6af1758c"],"request-id":["20b4b32a-9337-99eb-81e9-84ae6af1758c"],"Strict-Transport-Security":["max-age=31536000"],"X-FRAME-OPTIONS":["SAMEORIGIN"],"MicrosoftSharePointTeamServices":["16.0.0.6309"],"X-Content-Type-Options":["nosniff"],"X-MS-InvokeApp":["1; RequireReadOnly"],"Cache-Control":["max-age=0, private"],"Date":["Mon, 20 Mar 2017 04:28:11 GMT"],"P3P":["CP="ALL IND DSP COR ADM CONo CUR CUSo IVAo IVDo PSA PSD TAI TELo OUR SAMo CNT COM INT NAV ONL PHY PRE PUR UNI""],"Server":["Microsoft-IIS/8.5"],"X-AspNet-Version":["4.0.30319"],"X-Powered-By":["ASP.NET"]} at System.Activities.Statements.Throw.Execute(CodeActivityContext context) at System.Activities.CodeActivity.InternalExecute(ActivityInstance instance, ActivityExecutor executor, BookmarkManager bookmarkManager) at System.Activities.Runtime.ActivityExecutor.ExecuteActivityWorkItem.ExecuteBody(ActivityExecutor executor, BookmarkManager bookmarkManager, Location resultLocation) Exception from activity Throw If Sequence Sequence TryCatch Sequence Microsoft.SharePoint.WorkflowServices.Activities.RetryForDurationPolicy HttpPost Sequence Microsoft.Office.Project.Server.WorkflowActivities.LeaveProjectStage Sequence Microsoft.Office.Project.Server.WorkflowActivities.ExitProjectStageGate Sequence Flowchart Sequence XXXX.WorkflowXaml_56a7612b_0f43_48a2_885e_f284aea2ae50

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Description : The workflow waits for check in, and the check-in period can be 1 month to a year. So this Project was waiting for check-in from 1 month and when the user did check-in the Workflow displayed the above error. After restarting the workflow it worked fine. Since I am unable to find what is the cause of this error.

Please help.