@taltsafrir Hey, have you checked if your version of ruby-gem compatible with Logstash and Opensearch which you send your data to?
Is it installed properly?
Have you disabled pipeline.ecs_compatibility in logstash.yml before sending logs to your opensearch cluster?
Can you share your configuration files regarding Logstash? Such as logstash.yml and your pipeline.conf in logstash/conf.d/