I have set up /usr/share/foreman/config/ignored_environments.yml to ignore most classes on import.

# https://www.theforeman.org/manuals/1.16/index.html#4.2.2Classes
:filters:
  - !ruby/regexp '/^(?!roles::)/'

On first run, or if there are changes, everything works as expected. The table with selectable and ignored classes is shown.

On second run, when there are no changes, an error message is returned:

Bitte entschuldigen Sie, etwas ist schief gelaufen (Sorry something went wrong) can't modify frozen String

Log shows:

RuntimeError
can't modify frozen String
/usr/share/foreman/app/controllers/concerns/foreman/controller/environments.rb:59:in `list_ignored'
/usr/share/foreman/app/controllers/concerns/foreman/controller/environments.rb:32:in `import_environments'
/usr/share/foreman/vendor/ruby/2.1.0/gems/actionpack-4.2.8/lib/action_controller/metal/implicit_render.rb:4:in `send_action'
/usr/share/foreman/vendor/ruby/2.1.0/gems/actionpack-4.2.8/lib/abstract_controller/base.rb:198:in `process_action'
/usr/share/foreman/vendor/ruby/2.1.0/gems/actionpack-4.2.8/lib/action_controller/metal/rendering.rb:10:in `process_action'
/usr/share/foreman/vendor/ruby/2.1.0/gems/actionpack-4.2.8/lib/abstract_controller/callbacks.rb:20:in `block in process_action'
/usr/share/foreman/vendor/ruby/2.1.0/gems/activesupport-4.2.8/lib/active_support/callbacks.rb:117:in `call'
/usr/share/foreman/vendor/ruby/2.1.0/gems/activesupport-4.2.8/lib/active_support/callbacks.rb:117:in `call'

It would be nice to have a list of "ignored" classes instead of the error.