This is a known issue with a specific task list process, and is caused by the process not having a field list assigned to it.
To resolve, please follow the steps below.
In the Admin Tool, click Menus.
Open the menu set where the issue is occurring.
On the left, click Actions.
On the right, right-click on the erroring task list process and click Disable.
On the right, click Add Item, Existing Process.
Change the drop-down filter to SelectSystem\Workflow.
Highlight System - User's Workflow Task List with a single click.
Click OK.
Right click on the newly added process, and click Rename.
Change the name to My Task List.
Click Apply, OK.