Cors: No 'Access-Control-Allow-Origin' header is present on the requested resource. Error happening only on requests of high resources

Question

I only get the error when i try to make a GET of good chunk of data from yahoofinance, when i do small requests it works good. The front is a react deployed on Vercel, and the backend is flask deployed on Heroku. This is what im doing:

Flask App:

from flask import Flask, request, jsonify
from flask_cors import CORS, cross_origin
from utils import *


app = Flask(__name__)
cors = CORS(app, resources={
            r"/api/*": {"origins": "*"}}, support_credentials=True)


@app.route('/')
@cross_origin()
def index():
    return "Home"


@app.route('/api', methods=['GET'])
@cross_origin()
def api():
    return{
        "tutorial": "Flask React Herokuu"
    }


@app.route('/api/minsmax', methods=['GET'])
@cross_origin(origin='https://jjtrading-rfripp2.vercel.app', headers=['Content-Type', 'Authorization'])
def prices():
    days_back = request.args.get('days_back')
    coins_quantity = request.args.get('coins_quantity')
    response = jsonify(get_coins_in_min_max(days_back, coins_quantity))
    return response

React:

import { useState } from "react";
import axios from "axios";
import "./MinsMax.css";
import CoinsExcluded from "./CoinsExluded";
export const MinsMax = () => {
  const [state, setState] = useState({});
  const [result, setResult] = useState();
  const handleSubmit = (event) => {
    event.preventDefault();
    axios
      .get(
        `https://jjtradingapi.herokuapp.com/api/minsmax?days_back=${state.daysBack}&coins_quantity=${state.coinsQuantity}`,
        /* {
          headers: {
            xhrFields: {
              withCredentials: true,
            },
            crossDomain: true,
            contentType: "application/json; charset=utf-8",
            "Access-Control-Allow-Origin": "*",
          },
        } */
      )
      .then((result) => {
        console.log(result.data);
        setResult(result.data);
      });
    handleReset();
  };
  return(<div></div>)
};

The GET is succesfull when i do like 5 coins and a few days, but when i start to do more,i get this CORS error:

On the console:

Access to XMLHttpRequest at 'https://jjtradingapi.herokuapp.com/api/minsmax?days_back=2d&coins_quantity=50' from origin 'https://jjtrading-rfripp2.vercel.app' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

VM26:1          GET https://jjtradingapi.herokuapp.com/api/minsmax?days_back=2d&coins_quantity=50 net::ERR_FAILED 503

And in network:

Request URL: https://jjtradingapi.herokuapp.com/api/minsmax?days_back=2d&coins_quantity=50
Request Method: GET
Status Code: 503 
Referrer Policy: strict-origin-when-cross-origin
Cache-Control: no-cache, no-store
Connection: keep-alive
Content-Length: 506
Content-Type: text/html; charset=utf-8
Date: Sat, 26 Feb 2022 11:32:24 GMT
Server: Cowboy
Accept: application/json, text/plain, */*
Accept-Encoding: gzip, deflate, br
Accept-Language: es-419,es;q=0.9,en;q=0.8
Connection: keep-alive
Host: jjtradingapi.herokuapp.com
Origin: https://jjtrading-rfripp2.vercel.app
Referer: https://jjtrading-rfripp2.vercel.app/

The other routes that are simple,they work good. Any idea what can be the issue? Thanks so much in advance :)

Edit: maybe a function that has Try Except code makes it break? (happens when i request 50+ coins and when i get the CORS error). When i request to local host,i get the exceptions and the GET returns 200,but when i request to the heroku deploy, 503 with the CORS error. This is the function with the Try Except:

def get_mins_max_list(timeframe, amount_of_coins):
    result = {
        "mins": [],
        "max": [],
    }
    today = {
        "mins": None,
        "max": None,
        "tracked": 0,
        'errors': []
    }
    coins = getSymbols(amount_of_coins)
    coins_usd = symbols_usd(coins)
    for coin in coins_usd:
        try:
            minmax = get_min_max_price(coin, timeframe)
            result['mins'].append(minmax['min'])
            result['max'].append(minmax['max'])
            today['tracked'] += 1
        except IndexError:
            today['errors'].append(coin)
            continue
    today['mins'] = filter_today(result['mins'])
    today['max'] = filter_today(result['max'])
    return today