fix

app/models/external_user.rb

@@ -5,7 +5,7 @@ class ExternalUser < ActiveRecord::Base
   validates :external_id, presence: true
 
   def displayname
-    result =  "User " + user_id
+    result =  "User "
     if(!consumer.nil? && consumer.name == 'openHPI')
       result = Rails.cache.fetch("#{cache_key}/displayname", expires_in: 12.hours) do
          Xikolo::UserClient.get(external_id.to_s)[:display_name]