Hey @magnusbaeck
This thread (Unexpected pool error) solved the "Unexpected Pool Error".
It is now showing following message.
[2018-09-04T20:57:28,485][WARN ][logstash.outputs.elasticsearch] Marking url as dead. Last error: [LogStash::Outputs::Elasticsearch::HttpClient::Pool::HostUnreachableError] Elasticsearch Unreachable: [http://192.168.75.204:9200/][Manticore::SocketException] Connection refused: connect {:url=>http://192.168.75.204:9200/, :error_message=>"Elasticsearch Unreachable: [http://192.168.75.204:9200/][Manticore::SocketException] Connection refused: connect", :error_class=>"LogStash::Outputs::Elasticsearch::HttpClient::Pool::HostUnreachableError"}
[2018-09-04T20:57:28,516][ERROR][logstash.outputs.elasticsearch] Attempted to send a bulk request to elasticsearch' but Elasticsearch appears to be unreachable or down! {:error_message=>"Elasticsearch Unreachable: [http://192.168.75.204:9200/][Manticore::SocketException] Connection refused: connect", :class=>"LogStash::Outputs::Elasticsearch::HttpClient::Pool::HostUnreachableError", :will_retry_in_seconds=>2}