Symptoms

Assume in your solution you are using sub form to action on some specific task (e.g. approve/reject) and you action on the task by clicking a button on sub form.
You may notice that your solution does not work when action on sub form is being performed by OOF delegate user whereas primary destination user can action on the task without any issues errors.
 

Diagnoses

This is expected behavior if you not ensured in your solution handling of SharedUser parameter to pass it from main form to sub form.
This is related with the fact that two different API calls are executed when opening worklistitems - OpenWorklistItem (normal method) and OpenWorklistItemShared. And these need to be catered for during the Design and Building phase of your solution.
When using sub-form to action on the task with OOF delegate user shared user property not being added automatically into action URL (as it does for main form) causing inability to action on the task for OOF delegate user.
If SharedUser parameter added manually on sub view and parent form and then sub view parameter is populated by form parameter you will get correct behavior.

 

Resolution

At the moment SharedUser property is not being automatically transferred from a Parent Form to a Child Form - this is by design. There is an existing feature request to add better handling of this scenario in the future versions of K2 (TFS 646897).
In current versions of K2, you have to add to the all necessary FormsViews SharedUser parameter on sub view and parent form and then populate sub view parameter by form parameter.