After upgrading to a new version with an existing database, you encounter something like:
Caught MultipleObjectsReturned while rendering: get() returned more than one CacheKey -- it returned 12! Lookup parameters were {'key': 'cms-menu_nodes_en_1_1_user', 'language': 'en', 'site': 1L}
What has happened is that your database contains some old cache data in the menus_cachekey table. Just delete all those entries.