PayPal Sandbox get access token api is refusing work from my java code

Question

I have tried to access the REST api in order to get a PayPal access token, but something doesn't work there. I keep on getting code 403 (Forbidden) from the code.

The code is based on Jersey Jax-RS implmentation that's available on the Jakarta platform 9.1.0. Here's how it looks like:

try {
            Invocation.Builder builder = client.target("https://api-m.sandbox.paypal.com/")
                        .path("v1/oauth2/token")
                        .request()
                        .accept("application/json")
                        .acceptLanguage("en_US")
                        .header("Authorization", this.httpHeaders.get("Authorization"))
                            .header("Content-Type", "application/x-www-form-urlencoded");
            response = builder.post(Entity.entity("grant_type=client_credentials", MediaType.APPLICATION_FORM_URLENCODED_TYPE), Response.class);
        } catch (Exception e) {
            e.printStackTrace();
        }

the this.httpHeaders.get("Authorization") is where I stored the standard 'Basic ' + the base64 representation if the username:password.

In order to verify it's not an issue with my firewall i accessed the same api from windows 10 command line via cUrl and it works just find (I get a 200 ok reponse with a proper json block):

curl -v -X POST "https://api-m.sandbox.paypal.com/v1/oauth2/token" \
    -u "<CLIENT_ID>:<CLIENT_SECRET>" \
    -H "Content-Type: application/x-www-form-urlencoded" \
    -d "grant_type=client_credentials"  

Obviously I am doing something wrong here. I would appreciate your help to tell me what is needed to be fixed.