核心数据executeFetchRequest抛出NSGenericException（集合在枚举时发生了变异）

I'm developing a iPhone app with Core Data. All user data should be synchronized with our servers. For this purpose I created a subclass of NSOperation witch loads new data from our web service and creates corresponding managed objects. To maintain the relationships between them, every object is transmitted with a remoteID (which is the primary key of the relational server DB).I'm developing a iPhone app with Core Data. All