I didn't executed ./filebeat setup.
I have googled the error String index out of range: 0" when using include_fields processor and I have some news. I test since 2 hours, very strange issue.
When I use 7.6.1, if I change %{[beat.version]} to %{[agent.version]}, filebeat sends logs to elk,
I see the new index name filebeat-test-7.6.1-2020.03.19 and I see the number of documents increasing
but I can't research data for my server.
In "discover" section, I don't see my server.
I have some mapping conflict with the version of this agent but why I can't find my server ?
I already have mapping conflict with another index and I had the possibility to search with this warning.
Other test :
I have removed filebeat 7.6.1 and deleted the directories : /usr/share/filebeat/ /var/lib/filebeat/
I have installed filebeat 6.8.3 and now I can start filebeat.
It creates the index name (for a while, it's filebeat-test-2020.03.19) but I have no documents 
when I run ./filebeat -e -d "*"
I don't see error, I think it sent logs to elk, here an example :
I2020-03-19T15:47:14.800+0100    DEBUG   [publish]       pipeline/processor.go:309       Publish event: {
  "@timestamp": "2020-03-19T14:47:14.796Z",
  "@metadata": {
    "beat": "filebeat",
    "type": "doc",
    "version": "6.8.3",
    "pipeline": "filebeat-6.8.3-system-syslog-pipeline"
  },
  "beat": {
    "name": "server",
    "hostname": "server",
    "version": "6.8.3"
  },
  "log": {
    "file": {
      "path": "/var/log/messages-20200301"
    }
  },
  "prospector": {
    "type": "log"
  },
  "input": {
    "type": "log"
  },
  "fileset": {
    "module": "system",
    "name": "syslog"
  },
  "event": {
    "dataset": "system.syslog"
  },
  "host": {
    "containerized": false,
    "architecture": "x86_64",
    "os": {
      "name": "CentOS Linux",
      "codename": "Core",
      "platform": "centos",
      "version": "8 (Core)",
      "family": "redhat"
    },
    "id": "f42914dcfdb74052bd6026f96c682efc",
    "name": "server"
  },
  "source": "/var/log/messages-20200301",
  "offset": 11903077,
  "message": "Feb 27 09:28:00 tlinf006 influxd[3833]: [httpd] 127.0.0.1 - metricsesxuser [27/Feb/2020:09:28:00 -0500] \"POST /write?db=metricsesx HTTP/1.1\" 204 0 \"-\" \"Telegraf/1.12.4\" 5b084eee-596d-11ea-b586-000c29a29d52 976"
}
Sometime, I have this :
2020-03-19T15:47:14.804+0100    DEBUG   [publish]       pipeline/client.go:201  Pipeline client receives callback 'onDroppedOnPublish' for event: %+v{2020-03-19 15:47:14.800702215 +0100 CET m=+12.736744087 {"pipeline":"filebeat-6.8.3-system-syslog-pipeline"} {"beat":{"hostname":"server_name","name":"server_name","version":"6.8.3"},"event":{"dataset":"system.syslog"},"fileset":{"module":"system","name":"syslog"},"host":{"architecture":"x86_64","containerized":false,"id":"f42914dcfdb74052bd6026f96c682efc","name":"server_name","os":{"codename":"Core","family":"redhat","name":"CentOS Linux","platform":"centos","version":"8 (Core)"}},"input":{"type":"log"},"log":{"file":{"path":"/var/log/messages-20200223"}},"message":"Feb 20 08:50:00 tlinf006 influxd[3833]: [httpd] 127.0.0.1 - metricsesxuser [20/Feb/2020:08:50:00 -0500] \"POST /write?db=metricsesx HTTP/1.1\" 204 0 \"-\" \"Telegraf/1.12.4\" e327b549-53e7-11ea-91fd-000c29a29d52 1332","offset":11808824,"prospector":{"type":"log"},"source":"/var/log/messages-20200223"} { false 0xc42026e340 /var/log/messages-20200223 11809035 2020-03-19 15:47:05.239440054 +0100 CET m=+3.175481921 -1ns log map[] 100902053-64768}}
2020-03-19T15:47:14.804+0100    DEBUG   [publish]       pipeline/client.go:201  Pipeline client receives callback 'onDroppedOnPublish' for event: %+v{2020-03-19 15:47:14.797354791 +0100 CET m=+12.733396655 null {"log":{"file":{"path":"/var/log/messages-20200308"}},"message":"Mar  5 08:13:40 tlinf006 influxd[3833]: [httpd] 127.0.0.1 - metricsesxuser [05/Mar/2020:08:13:40 -0500] \"POST /write?db=metricsesx HTTP/1.1\" 204 0 \"-\" \"Telegraf/1.12.4\" 218e9488-5ee3-11ea-9521-000c29a29d52 1161","offset":11731751,"source":"/var/log/messages-20200308"} { false 0xc42026e680 /var/log/messages-20200308 11731962 2020-03-19 15:47:05.240800594 +0100 CET m=+3.176842470 -1ns log map[] 100943419-64768}}
And when I look to elk server, I have mapping conflict. How it's possible ? I have other filebeat agent with 6.8.3 agent, so I don't understand.
Is it due to the previous installation of 7.6.1 ?
Thank you for your help.