I encountered same issue with Ruby exception with no message.
following is logstash log info.
following is configuration info.
When I get output, the "index_postfix" can not be parsed as a variable.
following is output event es_index field:
This problem is happened after I restart logstash, but sometimes it works, sometimes it returns ruby exception.
logstash version is 6.2.3.
Anyone can help ? Many thanks!