Even though the errors appears…the Broker will still be processing documents but these shouldn’t be appear in the logs normally and it is related to configuration issue.
Is your IS running in cluster connected to same Broker Server and name?
Please check these Empower articles and see if this help:
When the IS starts to poll for broker, this means that the connection between IS and Broker has been lost.
As soon as the polling is able to connect to the Broker again, the connection is re-established and the polling stops.
So you should check for timeouts, firewall-clearances and similar issues to work around this problem.