An attempt to save a Task Type fails with HTTP 400 error: "Failed to update task type [Task Type Name]"
This error is presented when the Task Display Name field parameter does not exist as a custom property (field) as seen below.
In the above example, the OBJECTID property does not exist as a Custom Property within the Task Type.
1. Add the OBJECTID field through the "Add Property" button.
2. Save the Task Type again.