Fetch Error Help

Hi,

I´ve been struggeling for a week but I cant get this to work. My sites are handling multiple API calls from multiple sources but there´s this one which I cant get to work (I´m defeated). Essentially I´m trying to build an authorisation flow where the user can use Swedish Bank ID to login to the site.

First of all the error message:
“result”:{“message”:“invalid json response body at https://testbed-eid.scrive.com/api/v1/transaction/new reason: Unexpected token < in JSON at position 0”,“name”:“FetchError”,“stack”:“”,“_elementoryError”:true},“exception”:true}

Then the code:
// BACK_END
import {fetch} from ‘wix-fetch’;
const base = “https://testbed-eid.scrive.com/api/v1”;

let access_token = “1234567-123.132-123123-123213”
let redirecturl = “https://www.mylandingpage.se/”;

let provider = “seBankID”;

let method = “auth”

export async function BackendAPICall() {
const body = {

redirectUrl : redirecturl,

provider : provider,

method : method

};

const response = await fetch(base + “/transaction/new”, {

method: “POST”,
body: Object.keys(body)
.map(key => encodeURIComponent(key) + “=” + encodeURIComponent(body[key]))
.join( “&”),
headers: {
“Content-Type”: “application/x-www-form-urlencoded;charset=UTF-8”,
“Authorization”: "Bearer " + access_token

},
mode : “cors”

})
. catch ((err) => {
let errorMsg = err;
console.log(errorMsg);
});
return handleResponse(response);
}

async function handleResponse(response) {
const json = await response.json();
if (response.status !== 200) {
throw new Error(json.errorMessage);
}
return json;
}

Then in the front-end I´m just having a button like this:

export function button15_click(event) {
BackendAPICall()
}