改回原来的

lib/plugins/acts_as_watchable/lib/acts_as_watchable.rb

@@ -10,7 +10,7 @@ module Redmine
         def acts_as_watchable(options = {})
           return if self.included_modules.include?(Redmine::Acts::Watchable::InstanceMethods)
           class_eval do
-            has_many :watchers,  :dependent => :delete_all
+            has_many :watchers, :as => :watchable, :dependent => :delete_all
             has_many :watcher_users, :through => :watchers, :source => :user, :validate => false
 
             scope :watched_by, lambda { |user_id|