I suggest to activate configuration option
accept_http_10
(bool) Handle incoming HTTP/1.0 and HTTP 0.9 requests. This is off by default, and not fully standards compliant. There is support for pre-HTTP/1.1 style connect logic, dechunking, and handling lack of client host iff
because php function file_get_contents() uses protocol http/1.0

I checked the code in Mediawiki 1.35.5 the current LTS version.
If one enables httpEngine curl there, one will likely run into the same problem.
It still has $this->curlOptions[CURLOPT_HTTP_VERSION] = CURL_HTTP_VERSION_1_0;

I think I solved it.

Envoy is a proxy at WMF

When I drop the --http1.0 option from the request, it works also from my production server:

I have Mediawiki installations on two servers, the test server and the production server.

It is possible the problem is related to php function file_get_contents which defaults to http/1.0 protocl_version.

I use php 5.6.23

My function getIndexName reads now:
public function getIndexName( $name, $type = false, $identifier = false ) {

        global $wgElasticaIndexPrefix;  //Alois
        if ( $type ) {
                $name .= '_' . $type;
        }
        if ( $identifier ) {
                $name .= '_' . $identifier;
        }
        if ( isset($wgElasticaIndexPrefix) ) {  //Alois 3 lines
          return $wgElasticaIndexPrefix.$name;
        }
        return $name;
}

I think this is up to Daniel, who is the creator of the extension, and has already claimed the task.

@EBernhardson, yes it seems to work.
I defined a prefix 'wwi_' and let the function retrn $prefix . $name

I think I have a fix.