Shopify app stopped working suddenly without any changes

[sitakanta]

My app has stopped working suddenly for existing shopify stores . When a user clicks on my app in their dashboard my app loads, but some error occurred comes up and then white screen loads. On debugging I found that the call to my apps /graphql endpoint is giving 403 error. Here is my server.js code for reference . One strange thing I noticed is that the app works fine upon reinstalling and on fresh installs . Can someone help me as to why the 403 could be coming. 

Also one thing I noticed is that my Shopify.Utils.loadCurrentSession is not calling loadCallback and directly giving undefined session. Something similar to this issue -> https://github.com/Shopify/shopify-api-js/issues/162

import "@babel/polyfill";
import dotenv from "dotenv";
import "isomorphic-fetch";
import createShopifyAuth, { verifyRequest } from "@shopify/koa-shopify-auth";
import Shopify, { ApiVersion } from "@shopify/shopify-api";
import Koa from "koa";
import next from "next";
import Router from "koa-router";
const mongoose = require("mongoose");
import {
  storeCallback,
  loadCallback,
  deleteCallback,
} from "../callbacks/sessionCallback";

const webhooksRegistrar = require("../webhooks/_webhooksRegistrar.js");
const webhookRouters = require("../webhooks/_webhookRouters.js");
const userRoutes = require("../routes/index.js");
const sequelize = require("./config.db");
import SessionModels from "../models/SessionModel.js";
const { Op } = require("sequelize");

sequelize.sync();
dotenv.config();
const port = parseInt(process.env.PORT, 10) || 8081;
const dev = process.env.NODE_ENV !== "production";
const app = next({
  dev,
});
const handle = app.getRequestHandler();
const mongoUrl = process.env.MONGO_URL || "mongodb://127.0.0.1:27017/test";
mongoose.connect(
  mongoUrl,
  {
    useNewUrlParser: true,
    useUnifiedTopology: true,
  },
  (err) => {
    if (err) {
      console.log("--> There was an error connecting to MongoDB:", err.message);
    } else {
      console.log("--> Connected to MongoDB");
    }
  }
);
Shopify.Context.initialize({
  API_KEY: process.env.SHOPIFY_API_KEY,
  API_SECRET_KEY: process.env.SHOPIFY_API_SECRET,
  SCOPES: process.env.SCOPES.split(","),
  HOST_NAME: process.env.HOST.replace(/https:\/\/|\/$/g, ""),
  API_VERSION: ApiVersion.October20,
  IS_EMBEDDED_APP: true,
  SESSION_STORAGE: new Shopify.Session.CustomSessionStorage(
    storeCallback,
    loadCallback,
    deleteCallback
  ),
});

app.prepare().then(async () => {
  const server = new Koa();
  const router = new Router();
  server.keys = [Shopify.Context.API_SECRET_KEY];
  //offline token
  server.use(
    createShopifyAuth({
      accessMode: "offline",
      prefix: "/install",
      async afterAuth(ctx) {
        const { shop, accessToken, scope } = ctx.state.shopify;
        const host = ctx.query.host;

        webhooksRegistrar(shop, accessToken);
        ctx.redirect(`/auth?shop=${shop}`);
      },
    })
  );
  //online token for embedded apps
  server.use(
    createShopifyAuth({
      async afterAuth(ctx) {
        const { shop, accessToken, scope } = ctx.state.shopify;
        const host = ctx.query.host;
        ctx.redirect(`/?shop=${shop}&host=${host}`);
      },
    })
  );
  const handleRequest = async (ctx) => {
    await handle(ctx.req, ctx.res);
    ctx.respond = false;
    ctx.res.statusCode = 200;
  };
  router.post("/webhooks", async (ctx) => {
    try {
      await Shopify.Webhooks.Registry.process(ctx.req, ctx.res);
      console.log(`Webhook processed, returned status code 200`);
    } catch (error) {
      console.log(`Failed to process webhook: ${error}`);
    }
  });
  router.post(
    "/graphql",
    verifyRequest({ returnHeader: true }),
    async (ctx, next) => {
      await Shopify.Utils.graphqlProxy(ctx.req, ctx.res);
    }
  );
  router.get("(/_next/static/.*)", handleRequest); // Static content is clear
  router.get("/_next/webpack-hmr", handleRequest); // Webpack content is clear
  router.get("(.*)", async (ctx) => {
    const shop = ctx.query.shop;
    ctx.res.setHeader(
      "Content-Security-Policy",
      `frame-ancestors https://${shop} https://admin.shopify.com;`
    );

    if (!shop) {
      return await handleRequest(ctx);
    }


    const offlinesession = await SessionModels.count({
      where: {
        shop_url: shop,
        isOnline: "false",
        [Op.and]: [
          {
            access_token: {
              [Op.ne]: null,
            },
          },
          {
            access_token: {
              [Op.ne]: "undefined",
            },
          },
        ],
      },
    });

    if (offlinesession == 0) {
      return ctx.redirect(`/install/auth?shop=${shop}`);
    } else {
      const session = await Shopify.Utils.loadCurrentSession(
        ctx.req,
        ctx.res,
        true
      );

      if (session && session.expires && session.expires <= new Date()) {
        return ctx.redirect(`/auth?shop=${shop}`);
      } else {
        return await handleRequest(ctx);
      }
    }
  });

  server.use(webhookRouters());
  server.use(userRoutes());
  server.use(router.allowedMethods());
  server.use(router.routes());
  server.listen(port, () => {
    console.log(`> Ready on http://localhost:${port}`);
  });
});