since the upgrade from 6.2 to 6.3 there are no log entries in /var/log/logstash/logstash-plain.log
Logpath in /etc/logstash/logstash.yml
path.logs: /var/log/logstash
with the new queue.drain:true in the same file
found the error in the (for me) new "queue.drain: true", missing whitespace (copy & paste from the web)