Why does this REST request work from every client except Talend API tester - api

Question

This is an example of a REST POST request that works everywhere except Talend API tester. It give a 500 error.
Here is how I set the request up in my applications.
URLConnection connection = new URL("https://" + authHost + "/connect/token").openConnection();
logger.info("Connection oppened");
// message contains the form data: key=value&key=value&key=value
String message = "password=" + password + "&grant_type=password&username=" + username +
"&client_id=foolid&scope=mouthwash";
logger.info(message);
// Setting header fields.
connection.setRequestMethod("POST");
connection.setDoOutput(true);
connection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
connection.setRequestProperty("Accept", "application/json");
connection.getOutputStream().write(message.getBytes("UTF-8"));
Here is a working example in Postman
POST /connect/token HTTP/1.1
Host: identityserver.uat.example.com
Content-Type: application/x-www-form-urlencoded
Accept: application/json
Cookie: ARRAffinity=6a37d2ecf3441e27913cb832c4b767c68cad0e45c8806b3c5344d1b52d57f67a; ARRAffinitySameSite=6a37d2ecf3441e27913cb832c4b767c68cad0e45c8806b3c5344d1b52d57f67a
Content-Length: 137
password=secret&grant_type=password&username=fool&client_id=foolid&scope=mouthwash
And here is what Talend say it sends, which gets the 500 error -- which I understand comes from the server.
POST /connect/token HTTP/1.1
Accept: application/json
Content-Length: 137
Content-Type: application/x-www-form-urlencoded
Host: identityserver.uat.example.com
password=secret&grant_type=password&username=fool&client_id=foolid&scope=mouthwash
What's happening here?