Error "An unexpected error has occurred ... you can use this page temporarily close Web Parts or remove personal settings"

Document created by butlerj Support on Nov 19, 2018
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When clicking on an email link to approve workflow task, the following error might occur:

Sorry, something went wrong
An unexpected error has occurred. 
Web Parts Maintenance Page. If you have permission, you can use this page temporarily close Web Parts or remove personal settings. For more information contact your site administrator.

Additionally, all workflows with Flexi task action in a site collection end with Error Occurred status and the following error message displays in the workflow status page:

Error is task. Object reference not set to an instance of an object

The following error is also logged in ULS logs:

System.NullReferenceException: Object reference not set to an instance of an object.   
 at Nintex.Workflow.HumanApproval.ApprovalContextUtil.AddApprovalContextData(NWContextDataCollection data, Approver approver, NWWorkflowContext parentContext, ContextDataOptions options, Boolean taskitemDependentOnly)   
 at Nintex.Workflow.HumanApproval.Message.Send(Approver approver, Message persistTo)   
 at Nintex.Workflow.HumanApproval.MultiOutcomeTask.ClosePendingApprovers(Boolean informPendingApprovers, Outcome reason, UpdateApproverTaskMode closureMethod)   
 at Nintex.Workflow.Activities.MultiOutcomeInternal.TaskCompleted(Object sender, EventArgs e)   
 at System.Workflow.ComponentModel.Activity.RaiseEvent(DependencyProperty dependencyEvent, Object sender, EventArgs e)   
 at System.Workflow.Activities.ReplicatorActivity.Cancel(ActivityExecutionContext executionContext)   
 at System.Workflow.ComponentModel.CompositeActivity.HandleFault(ActivityExecutionContext executionContext, Exception exception)   
 at System.Workflow.ComponentModel.ActivityExecutor`1.HandleFault(T activity, ActivityExecutionContext executionContext, Exception exception)   
 at System.Workflow.ComponentModel.ActivityExecutor`1.HandleFault(Activity activity, ActivityExecutionContext executionContext, Exception exception)   
 at System.Workflow.ComponentModel.ActivityExecutorOperation.Run(IWorkflowCoreRuntime workflowCoreRuntime)   
 at System.Workflow.Runtime.Scheduler.Run()
Restore EditForm.aspx.
This error is the result of the EditForm.aspx having been deleted from WebPart Maintenance page for Workflow Task list.
It might also be permission related. For more information, see Web Parts Maintenance Page - Approver task form, deleted web part.