Iron-ajax 401 unauthorized or CORS issue

I'm having issues using iron-ajax on Polymer 2.0. My code's based on a Polymer 1.0 one and I'm trying to adapt it. I send my form through a POST like this:

Template:

        <div class="wrapper-btns">
            <paper-button raised class="primary" on-tap="postLogin">Log In</paper-button>
            <paper-button class="link" on-tap="postRegister">Sign Up</paper-button>
        </div>

Code:

    _setReqBody() {
        this.$.registerLoginAjax.body = this.formData;
    }

    postLogin() {
        this.$.registerLoginAjax.url = 'http://localhost:3001/sessions/create';
        this._setReqBody();
        this.$.registerLoginAjax.generateRequest();
    }

Iron-Ajax setup:

    <iron-localstorage name="user-storage" value="{{storedUser}}"></iron-localstorage>
    <app-data key="userData" data="{{storedUser}}"></app-data>

    <iron-ajax
        id="registerLoginAjax"
        method="post"
        content-type="application/json"
        handle-as="text"
        on-response="handleUserResponse"
        on-error="handleUserError"></iron-ajax>

And when I do I get the following error:

POST http://localhost:3001/sessions/create 400 (Bad Request)

And when I use this line on the Iron-Ajax :

with-credentials="true"

The error as it seems is a CORS problem:

XMLHttpRequest cannot load http://localhost:3001/sessions/create. Response to preflight request doesn't pass access control check: The value of the 'Access-Control-Allow-Origin' header in the response must not be the wildcard '*' when the request's credentials mode is 'include'. Origin 'http://127.0.0.1:8081' is therefore not allowed access. The credentials mode of requests initiated by the XMLHttpRequest is controlled by the withCredentials attribute.

What am I doing wrong?

Change the server-side code for the http://localhost:3001/sessions/create backend to send the response header Access-Control-Allow-Origin: http://127.0.0.1:8081/ in responses for requests from http://127.0.0.1:8081/, rather than sending back the response header Access-Control-Allow-Origin: * as it’s doing now.

The Credentialed requests and wildcards section of the MDN page on CORS explains why:

When responding to a credentialed request, the server must specify an origin in the value of the Access-Control-Allow-Origin header, instead of specifying the "*" wildcard.