Redmine

On our Redmine 3.2 installation we recognized that after some (not identifiable) moment the "Custom Fields" Entry in "administration" leads to an

Internal error

An error occurred on the page you were trying to access.
If you continue to experience problems please contact your Redmine administrator for assistance.

If you are the Redmine administrator, check your log files for details about the error.

we hoped it was introduced by some upgrade migration problem and updated to Redmine 3.3.1 today - and it is still the same.

In the Logs is happening the following:

Started GET "/custom_fields" for 123.123.123.123 at 2016-10-19 12:30:40 +0200                                                                                                                                        │
Processing by CustomFieldsController#index as HTML                                                                                                                                                                 │
  Current user: username (id=7)                                                                                                                                                                                       │
Completed 500 Internal Server Error in 19ms (ActiveRecord: 7.1ms)                                                                                                                                                  │
                                                                                                                                                                                                                   │
ActiveRecord::SubclassNotFound (The single-table inheritance mechanism failed to locate the subclass: 'OrderCustomField'. This error is raised because the column 'type' is reserved for storing the class in case │
of inheritance. Please rename this column if you didn't intend it to be used for storing the inheritance class or overwrite CustomField.inheritance_column to use another column for that information.):           │
  app/controllers/custom_fields_controller.rb:29:in `block (2 levels) in index'                                                                                                                                    │
  app/controllers/custom_fields_controller.rb:27:in `index'                                                                                                                                                        │
  lib/redmine/sudo_mode.rb:63:in `sudo_mode'

what could have caused this?