GDAX Post Call returns invalid signature

I am trying to make a post request on GDAX. But I always receive a "invalid signature" message. GDAX API Docs for creating request + signing: https://docs.gdax.com/#creating-a-request

Preshash string returns the following:

1500627733POST/orders{"price":"1000.0","size":"0.02","type":"limit","side":"sell","product_id":"BTC-EUR"}

My signature method:

public String generateSignature(String requestPath, String method, String body, String timestamp) {
        try {
            String prehash = timestamp + method.toUpperCase() + requestPath + body;
            byte[] secretDecoded = Base64.getDecoder().decode(secretKey);
            SecretKeySpec keyspec = new SecretKeySpec(secretDecoded, "HmacSHA256");
            Mac sha256 = (Mac) Mac.getInstance("HmacSHA256").clone();
            sha256.init(keyspec);
            return Base64.getEncoder().encodeToString(sha256.doFinal(prehash.getBytes()));
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

My request method:

private boolean placeLimitOrder(String currencyPair, String side, String price, String size)
            throws UnirestException {

        String timestamp = Instant.now().getEpochSecond() + "";
        String api_method = "/orders";
        String path = base_url + api_method; //base_url = https://api.gdax.com
        String method = "POST";
        String b = "{\"price\":\"1000.0\",\"size\":\"0.02\",\"type\":\"limit\",\"side\":\"sell\",\"product_id\":\"BTC-EUR\"}";
        JsonNode n = new JsonNode(b);
        String sig = generateSignature(api_method, method,b, timestamp);

        HttpResponse<JsonNode> rep = Unirest.post(path).header("accept", "application/json")
                .header("content-type", "application/json")
                .header("CB-ACCESS-KEY", publicKey)
                .header("CB-ACCESS-PASSPHRASE", passphrase)
                .header("CB-ACCESS-SIGN", sig)
                .header("CB-ACCESS-TIMESTAMP", timestamp)
                .body(n)
                .asJson();

        System.out.println(rep.getStatusText()); //Bad Request

        System.out.println(rep.getBody().toString()); //invalid signature

        System.out.println(sig); //returns something


        return false;
    }

I also tried to make a API Request Call with Insomnia but it returns the same message ("invalid signature").

Any clues?

Thank you very much in advance!