Angular + Expressjs Http failure response Unknown error 0

Question

I'm using Angular 8 and ExpressJS. Everything work perfectly, the app is live and a lot of users (6000) are using it. But there are a few users (less than 10) that get Unknown Error (code 0) from any API call.

This is the error response:

{"headers":{"normalizedNames":{},"lazyUpdate":null,"headers":{}},"status":0,"statusText":"Unknown Error","url":"https:/xxxxxxxxxx/api/auth/recover-pass","ok":false,"name":"HttpErrorResponse","message":"Http failure response for https://xxxxxxxx/api/auth/recover-pass: 0 Unknown Error","error":{"isTrusted":true}}

I readed it can be an CORS error, but I don't think so. I'm using this

app.use((req, res, next) => {
   res.setHeader("Access-Control-Allow-Methods", "POST, PUT, OPTIONS, DELETE, GET");
   res.header("Access-Control-Allow-Origin", "*");
   res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept, x-access-token");
   res.header("Access-Control-Allow-Credentials", true);
   next();
});