The main list item form works fine. But in my workflow, when anyone tries to complete a task in the task form, we get Object Reference not set to instance of an object when we try to submit the task. The form loads, we can use it, it's just when we go to save it. We have not changed our workflow in months, and this happened suddenly only a few days ago. 

When I look at the developer console, I get 500 (Internal Server Error) with reference to wus-srv-ufo.nintexo365.net/api/item?SPHostUrl=...

Our workaround has been to go into the task list and change the status to completed there. That allows the workflow to move forward, but it's a pain.

I've tried republishing the form and even recreating the task action and task form. Still get the same results.