So far this only appears to affect the "get_config" message. In the main bundle I use the same UserDefaults contructor and get this error:
Couldn't read values in CFPrefPlistSource<[...]> (Domain: [group id], User: kCFPreferencesAnyuser, ByHost: Yes, Container: (null), Contents Need Refresh: Yes): Using kCFPreferencesAnyUser with a container is only allowed for System Containers, detaching from cfprefsd
This is what led me to the Stackoverflow answer and the attempted fixes above. However this didn't solve the issue or remove this error message.