I have website on React and getting the callback with payment result from the payment engine to the backend (Node.js).

I receive the callback from payment engine using a POST method:

app.post("/api/payment-status", async (req, res) => {
  res.setHeader("Content-Type", "application/json");
  res.setHeader("Cache-Control", "no-cache, no-store, must-revalidate");
  const {createdDate, transactionStatus, reason, reasonCode, orderReference} = JSON.parse(Object.keys(req.body)[0]);
  console.log("transactionStatus: ", transactionStatus, "reason: ", reason, "reasonCode: ", reasonCode, "orderReference: ", orderReference);

  const wfpRes = await fetchItemSetting(res, webConfig.enableWfpSandboxID);
  const secretKey = isWebConfigOn(wfpRes[0].is_active) ? webConfig.wfpSandboxSecretKey! : webConfig.wfpSecretKey!;
  const wfpAcceptStatus = webConfig.wfpAcceptStatus;

  const wfpResData = {
    orderReference,
    status: wfpAcceptStatus,
    time: createdDate
  };

  const wfpData = generateWfpDataForSignature(wfpResData);
  const wfpSignature = getWfpSignature(secretKey, wfpData);
  let paymentData = {};

  if (transactionStatus === webConfig.wfpTransactionStatus && reason === webConfig.wfpReason && reasonCode === Number(webConfig.wfpReasonCode) && orderReference) {
      console.log("Success!!!");
      paymentData = {
        isSuccess: true,
        wfpOrderReference: orderReference,
        wfpStatus: transactionStatus,
        wfpReason: reason,
        msg: "Успішний платіж.",
        wfpReasonCode: reasonCode
      };
  } else {
      console.log("Failed!!!!");
      paymentData = {
        isSuccess: false,
        wfpOrderReference: orderReference,
        wfpStatus: transactionStatus,
        wfpReason: reason,
        msg: `Замовлення №${orderReference}. Код неуспішного платежу: ${reasonCode}!`,
        wfpReasonCode: reasonCode
      };
  }
  
  if (typeof paymentData === "object" && Object.keys(paymentData).length > 0) {
      eventEmitter.emit("paymentStatusReceived", paymentData);
  }

  return res.status(200).send({...wfpResData, signature: wfpSignature});
});

To get the result I use EventEmitter.

const EventEmitter = require("events");

const eventEmitter = new EventEmitter();

app.get("/api/order/payment-data", (req, res) => {
  res.setHeader("Content-Type", "application/json");
  res.setHeader("Cache-Control", "no-cache, no-store, must-revalidate");
  eventEmitter.once("paymentStatusReceived", (paymentData) => {
    if (typeof paymentData === "object" && Object.keys(paymentData).length > 0) {
        return res.json(paymentData);
    }
  });
});

Instead of paymentData object data I get the html output from 502 error page. When I add the following headers to axios GET request.

"Access-Control-Allow-Origin": "*",
 Accept: "application/json",
"Content-Type": "application/json",
"Access-Control-Expose-Headers": "*, Authorization",
"Access-Control-Allow-Headers": "*",
"Access-Control-Allow-Credentials": "true",
"Access-Control-Allow-Methods": "GET, POST, OPTIONS"

Then I get 406 Not Acceptable status code.

For example, the first time I pay in the test mode I get the correct data but when I try a few more times it just 406 error code. Or garbage html when I remove this content type: Accept: "application/json".

Any ideas how to find and fix this error? Or what should be the correct code to wait for a callback from a payment engine and then get it as JSON?