Hi Emre,
Can you try these set of steps:
1. Add 1 hour of ETC to one of the assignments, and Save
2. Change the Task Finish date to 10/18/2016, and Save
3. Check the box to make the task a 'Fixed Duration' task, and Save
4. Change the ETC back to 0 for the assignment, and Save
This will make the Task Finish date be read-only again, but it will keep the new date of 10/18/2016.
In order to be able to have a task with a Finish date later than the latest date that actuals were posted on (for tasks with 0 ETC remaining), the task needs to be a Fixed Duration task.
Note: If you don't have the 'Fixed Duration' checkbox configured on the Task Properties screen, you can add it to the Task list view and check the box for this task there.
I hope this helps!
Best Regards,
Maria Whiteside