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As Ganesh suggested, Task list in SP Online can be used as replacement for Project tasks in SP 2010.
Here is link to official documentation on connecting task list to project.
https://support.office.com/en-ie/article/create-and-manage-a-project-task-list-466ad207-46fd-4c77-9af1-41bc23cec21a#ID0EAACAAA=Online,_2016,_2013
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It may be the nature of your WF. Are you doing update to that task in wf? if so you need put a wait action after it. Sometimes WF is faster than sharepoint due to async calls and hits race condition. Put a 5 min wait after update of the task and see if that solves your problem.
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I had similar requirement and achieved by using the query like this-
http://PC:2010/site/_vti_bin/listdata.svc/listName?$select=ThumbnailImage,Created,ListingOrder&$orderby=(ListingOrder eq null)
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