This should be marked as a WARNING not an ERROR in your logs, as it means that Apple has closed the connection. The subsequent logging message indicates the library re-attempted to deliver the message (successfully otherwise you would have seen another error).
Apple usually drops the connection if one of the device tokens is invalid (e.g. old token, production token when using sandbox and visa versa). Please ensure that you send notifications to valid device tokens only.
Regards,- Mahmood