Message$Register was not delivered

I am using akka-http as a macro-service api endpoint, but the server sometimes seen like unresponsive while this error message occurs:

Message [$Register] from Actor[akka://iam-authority-service/system/StreamSupervisor-0/$$2RM#-1653270447] to Actor[akka://iam-authority-service/system/IO-TCP/selectors/$a/158454#-148630969] was not delivered. [3] dead letters encountered. If this is not an expected behavior, then [Actor[akka://iam-authority-service/system/IO-TCP/selectors/$a/158454#-148630969]] may have terminated unexpectedly, This logging can be turned off or adjusted with configuration settings ‘akka.log-dead-letters’ and ‘akka.log-dead-letters-during-shutdown’.

akka 2.5.27