when any eshuttle site created by a unique permission after that sharepoint asking for associated groups with that site.some times that site not able to get the associated groups owner, thats why when User try to approve the eshuttle task from client or sharepoint task window,user will get object Refrence Error.Here is Quick solution to fix this problem on sharepoint.you just follow these steps and your eshuttle site will work perfectly.
Check attachment to see the solution.
2514.eSHUTTLE_workflow_approve_error.docx