Elasticsearch not working [FATAL] FTS Server is down

Hi,

I got an error message:

[41104][1][FATAL] FTS Server is down, postponing the job for full index.

It works when I click test in Admin > Search > click test, after 1-2 minutes is not work again.

I have live-server and test-server but in test server works fine.

When I check with <url>:port in live-server, It returns result:

{

  "status" : 200,

  "name" : "<url>",

  "version" : {

    "number" : "1.3.1",

    "build_hash" : "<hash>",

    "build_timestamp" : "<date>",

    "build_snapshot" : false,

    "lucene_version" : "4.9"

  },

  "tagline" : "You Know, for Search"

}

SugarCRM Enterprise, Version 7.5.2.0

I try to investigate but no idea.

Do you have any idea to fix or investigate it?

Thank you,

M