Rails.logger.info(params[:question])
=> {"title"=>"katt"}

@question_list.questions.create(params[:question])
=> ActiveModel::ForbiddenAttributesError (ActiveModel::ForbiddenAttributesError)    

 @question_list.questions.create("title"=>"katt")
# SUCCES!

I cannot understand why Rails not accepts the params when the exact same value written by hand works fine?

Update
controller:

def new_question
  @question_list.questions.create(params[:question])
  render nothing: true
end

private
  def set_question_list
    @question_list = QuestionList.find(params[:id])
  end

  def question_list_params
    params.require(:question_list).permit(questions_attributes: [:id, :question_list_id, :title, :position, :_destroy])
  end

view:

<%= form_for @question_list, url: new_question_question_list_path, remote: true do |f| %>
  <%= f.text_field :title %>
  <%= f.submit %>
<% end %>