This little tool will help you to test if your Graylog server is able to reach the Graylog License API - the parameters of the trustStore and the proxy might be added and modified to fit your local needs.

URLConnectionReader.java

// Based on java example: http://docs.oracle.com/javase/tutorial/networking/urls/readingWriting.html
// save as: URLConnectionReader.java
// compile using JDK: javac URLConnectionReader.java
// run: java -Djavax.net.ssl.trustStore=/path/to/cacerts.jks -Dhttp.proxyHost=10.0.0.100 -Dhttp.proxyPort=8800 URLConnectionReader
//      if additional debugging is needed add -Djavax.net.debug=all to the above
// good path: returns HTML
// bad path: throws an exception

import java.net.*;
import java.io.*;

public class URLConnectionReader {
    public static void main(String[] args) throws Exception {
        URL oracle = new URL("https://api.graylog.com/");
        URLConnection yc = oracle.openConnection();
        BufferedReader in = new BufferedReader(new InputStreamReader(
                                    yc.getInputStream()));
        String inputLine;
        while ((inputLine = in.readLine()) != null)
            System.out.println(inputLine);
        in.close();
    }
}