Redmine

Hello,

I am not able to create a subtask.

Environment:
  Redmine version                3.1.1.stable
  Ruby version                   2.1.7-p400 (2015-08-18) [i386-mingw32]
  Rails version                  4.2.4
  Environment                    production
  Database adapter               Mysql2
SCM:
  Subversion                     1.8.5
  Git                            1.8.1
  Filesystem                     
Redmine plugins:
  clipboard_image_paste          1.10
  redmine_issue_reminder         1.0.0

Creating an issue (parent issue)
and Creating a subtask via the add subtask link.

the "subtask"

URL seems to be correct:
http://redmine.domain.com/projects/test/issues/new?issue[parent_issue_id]=1709&issue[tracker_id]=3
--> [parent_issue_id]=1709

But the sub task issue entry in the database has no parent_id:

INSERT INTO `issues` (`id`, `tracker_id`, `project_id`, `subject`, `description`, `due_date`, `category_id`, `status_id`, `assigned_to_id`, `priority_id`, `fixed_version_id`, `author_id`, `lock_version`, `created_on`, `updated_on`, `start_date`, `done_ratio`, `estimated_hours`, `parent_id`, `root_id`, `lft`, `rgt`, `is_private`, `closed_on`) VALUES
(1709, 3, 22, 'Parent Task', 'Sub Task Test', NULL, NULL, 1, NULL, 2, NULL, 1, 0, '2016-07-07 15:20:07', '2016-07-07 15:20:07', NULL, 0, NULL, NULL, 1709, 1, 2, 0, NULL),
(1710, 3, 22, 'Sub Task', 'Sub Task Test', NULL, NULL, 1, NULL, 2, NULL, 1, 0, '2016-07-07 15:22:28', '2016-07-07 15:22:28', NULL, 0, NULL, NULL, 1710, 1, 2, 0, NULL);

Finally there are no subtasks in the given project: