Yeah, found it. The problem comes from cancancan/lib/cancan/controller_resource.rb in 

    def resource_class
      case @options[:class]
      when false
        name.to_sym
      when nil
        namespaced_name.to_s.camelize.constantize
      when String
        @options[:class].constantize
      else
        @options[:class]
      end
    end

The error cames from namespaced_name.to_s.camelize.constantize, which resolves to "Authority". I now set

module Wobauth
  class AuthoritiesController < ApplicationController
    skip_load_and_authorize_resource
    load_and_authorize_resource class: Wobauth::Authority
...

This sets the class name manually. Seems to work now. Thank you for the idea ;-)