JSON response could not be decoded

Probably a wrong path.

Please follow the guide step by step:
https://www.wpsolr.com/guide/configuration-step-by-step-schematic/configure-your-indexes/connect-to-an-apache-solr-index/

Now SOLR is installed on my apache server (HTTP), and the index folder “new-index” exists in /usr/home/USERNAME/solr/server/solr.

Trying to connect occurs an error “We could not connect to your Solr server. It’s probably because the port is blocked. Please try another port, for instance 443, or contact your hosting provider/network administrator to unblock your port.

Solr HTTP error: HTTP request failed, Failed to connect to localhost port 8983: Connection refused”

Port 80 also fails, but the notice is another: “We could not join your Solr server. Your Solr path could be malformed, or your Solr server down (Solr code 404)”.

Do I have to put the account owner’s user name and password into the fields key and secret of TAB 0?

What is your Solr path in wpsolr index settings ?

I tried it with

Host: localhost and IP-address and Host-Name
the moment
Solr index path: /solr/new-index
and /solr/server/solr/new-index
and /usr/home/USERNAME/solr/server/solr/new-index
and /usr/home/USERNAME/solr/new-index

Correct setting:

Host: localhost
Scheme: HTTP
Port: 8983
Solr index path: /solr/new-index

You must ensure you can access your Solr admin UI locally.

Can you show a capture of your Solr admin, menu “Core admin” ?
(hide your external url)

Now the connection is successful established.
Thank you!

Have fun!