Problem
You get the following error in redmine when trying to update a task that has a parent task.

Parent task translation missing: en, activerecord, errors, models, issue, attributes, parent_issue_id, not_a_valid_parent

Solution
It seems that for some reason when you add more than one subtask, the lft and rgt id numbers are not created correctly so you cannot update the subtasks.
If the numbers are overlapping (rgt for first subtask 2 and lft for second subtask 2), then there is a problem.
So in order to be able to solve that you will need to change the lft, rgt ids to have subsequent numbers as:
first subtask: lft -> 1 , rgt -> 2
second subtask: lft -> 3, rgt -> 4