Redmine

Hi Redmine users,

I have a problem when I am trying to make a new issue: when i make a new issue and assign the issue to a user and make the issue redmine does not save the assigned user.
Does anyone have an idea what could be the problem?
I already checked my permissions and everything is normal and should be working. The "Issues can be assigned to this role" checkbox is checked for all of my roles.

Redmine info:

Environment:
  Redmine version                3.3.2.stable
  Ruby version                   2.3.1-p112 (2016-04-26) [x86_64-linux-gnu]
  Rails version                  4.2.7.1
  Environment                    production
  Database adapter               PostgreSQL
SCM:
  Subversion                     1.9.3
  Git                            2.7.4
  Filesystem                     
Redmine plugins:
  redmine_bootstrap_kit          0.2-devel
  redmine_checklists             3.1.5
  redmine_contacts               4.0.6
  redmine_contacts_helpdesk      3.0.7
  redmine_default_group          0.0.1
  redmine_ezsidebar              0.1.1
  redmine_import_issues          1.0.0
  redmine_issue_templates        0.1.4
  redmine_jenkins                1.0-devel
  redmine_my_page_queries        2.1.7
  redmine_questions              0.0.7
  redmine_slack                  0.2
  scrum                          0.16.2

Production.log log for when I make a new issue:

Started POST "/support//projects/chessol/issues" for 10.50.24.20 at 2017-05-02 16:46:04 +0200
Processing by IssuesController#create as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"------", "form_update_triggered_by"=>"", "issue"=>{"is_private"=>"1", "tracker_id"=>"1", "subject"=>"Test Issue", "description"=>"Test Issue", "status_id"=>"1", "priority_id"=>"2", *"assigned_to_id"=>"6"*, "parent_issue_id"=>"", "start_date"=>"2017-05-02", "due_date"=>"", "estimated_hours"=>"", "done_ratio"=>"0", "custom_field_values"=>{"12"=>"", "23"=>""}, "deals_issue_attributes"=>{"deal_id"=>""}, "sprint_id"=>"1", "pending_effort"=>"0"}, "issue_template"=>"", "template_search_filter"=>"", "was_default_status"=>"1", "commit"=>"Create", "project_id"=>"------"}
  Current user: ------ (id=6)
  Rendered mailer/_issue.text.erb (1.1ms)
  Rendered mailer/issue_add.text.erb within layouts/mailer (1.6ms)
  Rendered mailer/_issue.html.erb (1.1ms)
  Rendered mailer/issue_add.html.erb within layouts/mailer (1.4ms)
Redirected to ------/support/issues/2413
Completed 302 Found in 147ms (ActiveRecord: 12.8ms)