"Content-Type header [text/plain; charset=ISO-8859-1] is not supported","status":406

Hello together,

I am new to the elastic stack and have no further experience than just google a solution. In my environment I have to use Logstash on a remote server. I have no control and influence over the version which is installed. Currently it is Logstash 1.5.4 which I have to use. My current elasticsearc version is 7.9.3.

The following content are my current output lines of my logstash configuration:

output {
		elasticsearch {
			protocol => http
			ssl => true
			cacert => "pathToCertificate"
			host => "url:port"
			user => "myuser"
			password => mysecretpassword
			cluster => "clustername"
			index => "classic-dev-%{+YYYY.MM.dd}"
			
		}
}

Running this config file I get the following error:

Failed to install template: [406] {"error":"Content-Type header [text/plain; charset=ISO-8859-1] is not supported","status":406} {:level=>:error}

Can somebody please help me with this error?

Thanks for your help in advance.

Can you provide the 3 INFO messages that precede that? That will confirm where exactly this error is occurring.

These are the only log files which are shown to me for the error:

Failed to install template: [406] {"error":"Content-Type header [text/plain; charset=ISO-8859-1] is not supported","status":406} {:level=>:error}
Logstash startup completed
Got error to send bulk of actions: [406] {"error":"Content-Type header [text/plain; charset=ISO-8859-1] is not supported","status":406} {:level=>:error}
Failed to flush outgoing items {:outgoing_count=>312, :exception=>"Elasticsearch::Transport::Transport::Errors::NotAcceptable", ...