Output to Elasticsearch - no living connections to connection pool

This is a brand new ELK 5.0 install and I am a newbie. ES, Logstash, Kibana and Metricbeat (localhost metrics) installed on single server with xpack local authentication enabled and working as expected.. Attempting to collect syslogs from remote servers where filebeat is installed and sending to ELK server. Logstash is show records received from input and are successfully passed through the filter but nothing is going to output channel to ES. Every minute, the following message is logged in logstash log (/var/log/logstash/logstash-plain.log ):

[2016-11-02T07:02:43,814][ERROR][logstash.outputs.elasticsearch] Attempted to send a bulk request to elasticsearch, but no there are no living connections in the connection pool. Perhaps Elasticsearch is unreachable or down? {:error_message=>"No Available connections", :class=>"LogStash::Outputs::ElasticSearch::HttpClient::Pool::NoConnectionAvailableError", :will_retry_in_seconds=>64}

Nothing pertinent in ES log. Suggestions on where to investigate would be appreciated.

OS and ES versions

Ubuntu 16.04.1 LTS
ES v5.0.0
LS v5.0.0


input {
beats {
port => 5044
ssl => true
ssl_certificate => "/etc/pki/tls/certs/logstash-forwarder.crt"
ssl_key => "/etc/pki/tls/private/logstash-forwarder.key"
filter {
if [type] == "syslog" {
grok {
match => { "message" => "%{SYSLOGTIMESTAMP:syslog_timestamp} %{SYSLOGHOST:syslog_hostname} %{DATA:syslog_program}(?:[%{POSINT:syslog_pid}])?: %{GREEDYDATA:syslog_message}" }
add_field => [ "received_at", "%{@timestamp}" ]
add_field => [ "received_from", "%{host}" ]
syslog_pri { }
date {
match => [ "syslog_timestamp", "MMM d HH:mm:ss", "MMM dd HH:mm:ss" ]
output {
elasticsearch {
hosts => ["localhost:9200"]
user => "logstash_internal"
password => "password"
sniffing => true
manage_template => false
index => "%{[@metadata][beat]}-%{+YYYY.MM.dd}"
document_type => "%{[@metadata][type]}"

dumb questions: does a normal curl localhost:9200 work? can the current terminal session have an HTTP_PROXY environment variable set?

Normal monitoring API calls via curl commands to ES (port 9200) and LS (9600) work just fine. Found nothing that indicates quere buildups or memory shortages. Not found an API to query these HTTP connections though.