Missing partial wiki_extensions/_html_header

[a1s]

I am moving a Redmine server from v1.3.0 to v3.4.4 (that is the version currently shipped with Ubuntu 18.04.3 LTS)

I have installed Wiki Extensions plugin v0.8.2 which is said to be compatible with Redmine v3.4.

When I try to access any page, I get this error:

Completed 500 Internal Server Error in 84ms (ActiveRecord: 47.0ms)

ActionView::Template::Error (Missing partial wiki_extensions/_html_header with {:locale=>[:en], :formats=>[:html], :variants=>[], :handlers=>[:erb, :builder, :raw, :ruby, :rsb]}. Searched in:
  * "/usr/share/redmine/app/views"
):
    13: <%= stylesheet_link_tag 'rtl', :media => 'all' if l(:direction) == 'rtl' %>
    14: <%= javascript_heads %>
    15: <%= heads_for_theme %>
    16: <%= call_hook :view_layouts_base_html_head %>
    17: <!-- page specific tags -->
    18: <%= yield :header_tags -%>
    19: </head>
  lib/redmine/hook/view_listener.rb:59:in `block (2 levels) in render_on'
  lib/redmine/hook/view_listener.rb:57:in `map'
  lib/redmine/hook/view_listener.rb:57:in `block in render_on'
  lib/redmine/hook.rb:61:in `block (2 levels) in call_hook'
  lib/redmine/hook.rb:61:in `each'
  lib/redmine/hook.rb:61:in `block in call_hook'
  lib/redmine/hook.rb:58:in `tap'
  lib/redmine/hook.rb:58:in `call_hook'
  lib/redmine/hook.rb:96:in `call_hook'
  app/views/layouts/base.html.erb:16:in `_app_views_layouts_base_html_erb___1210063697575443044_47424541354420'
  app/controllers/application_controller.rb:491:in `block (2 levels) in render_error'
  app/controllers/application_controller.rb:489:in `render_error'
  app/controllers/application_controller.rb:477:in `render_404'
  app/controllers/application_controller.rb:501:in `missing_template'

What could be the problem? Did I miss some important setting?