部署完 Redmine 后，新建中文的用户，提示 500 错误

日志：

ActionView::Template::Error (incompatible character encodings: UTF-8 and ASCII-8BIT):
    6: <fieldset class="box tabular">
    7:   <legend><%=l(:label_information_plural)%></legend>
    8:   <p><%= f.text_field :login, :required => true, :size => 25  %></p>
    9:   <p><%= f.text_field :firstname, :required => true %></p>
    10:   <p><%= f.text_field :lastname, :required => true %></p>
    11:   <p><%= f.text_field :mail, :required => true %></p>
    12:   <% unless @user.force_default_language? %>
  lib/redmine/views/labelled_form_builder.rb:31:in `text_field'
  app/views/users/_form.html.erb:9:in `_app_views_users__form_html_erb___1339523949848742532_70169066250380'
  app/views/users/_general.html.erb:2:in `block in _app_views_users__general_html_erb__1484988381539933793_58512080'
  app/helpers/application_helper.rb:1056:in `labelled_form_for'
  app/views/users/_general.html.erb:1:in `_app_views_users__general_html_erb__1484988381539933793_58512080'
  app/views/common/_tabs.html.erb:22:in `block in _app_views_common__tabs_html_erb__1558437477300092939_57895140'
  app/views/common/_tabs.html.erb:21:in `each'
  app/views/common/_tabs.html.erb:21:in `_app_views_common__tabs_html_erb__1558437477300092939_57895140'
  app/helpers/application_helper.rb:328:in `render_tabs'
  app/views/users/edit.html.erb:9:in `_app_views_users_edit_html_erb__748188076281410297_58219660'

环境： Ubuntu14.04 Redmine2.6.0 Ruby2.0.0 Mysql5.5.40

网上的方法试过一些，但不能解决问题。听说是要用 Ruby1.8 的版本。