Plugin port : routing error

I need to port some working plugins from EL to debian, foreman 3.3.
Therefore, I added gem to new file inside the bundler.d/ directory.
DB migration was successful, and foreman starts up successful. Login possible.
But when trying to access anything (dashboard, plugin-page) it end with error message:
No route matches {:action=>"index", :controller=>"foreman_configurator/hosts"}

In the logs, I have a backtrace, but that is 100+ lines and I don’t know what’s relevant.
I searched for docu, in the foreman manual and plugin dev guide.
Even I cannot find any documentation how plugis hook into foreman.

Here’s snippet from production.log

2022-09-29T08:50:18 [I|app|38d0fdf7] User 'admin' logged in from ''                                                                                                                                                    [120/1944]
2022-09-29T08:50:18 [I|app|38d0fdf7] Redirected to
2022-09-29T08:50:18 [I|app|38d0fdf7] Completed 302 Found in 1317ms (ActiveRecord: 202.7ms | Allocations: 42865)
2022-09-29T08:50:18 [I|app|dd04fc6c] Started GET "/" for at 2022-09-29 08:50:18 +0000
2022-09-29T08:50:18 [I|app|dd04fc6c] Processing by DashboardController#index as HTML
2022-09-29T08:50:18 [I|app|dd04fc6c]   Rendering dashboard/index.html.erb within layouts/application
2022-09-29T08:50:20 [I|app|dd04fc6c]   Rendered dashboard/index.html.erb within layouts/application (Duration: 1256.1ms | Allocations: 73853)
2022-09-29T08:50:20 [I|app|dd04fc6c]   Rendered layouts/_application_content.html.erb (Duration: 24.4ms | Allocations: 785)
2022-09-29T08:50:20 [I|app|dd04fc6c]   Rendering layouts/base.html.erb
2022-09-29T08:50:20 [I|app|dd04fc6c]   Rendered layouts/base.html.erb (Duration: 199.8ms | Allocations: 8234)
2022-09-29T08:50:20 [W|app|dd04fc6c] No route matches {:action=>"index", :controller=>"foreman_configurator/hosts"}
2022-09-29T08:50:20 [I|app|dd04fc6c] Backtrace for 'No route matches {:action=>"index", :controller=>"foreman_configurator/hosts"}' error (ActionView::Template::Error): No route matches {:action=>"index", :controller=>"foreman_configurator/hosts"}
 dd04fc6c | /usr/share/foreman/vendor/ruby/2.7.0/gems/actionpack `generate'