Json response "Unexpected token in JSON at position 0"

Question

I am writing login form using AMP form:

<form method="POST" class="p2 mb3" action-xhr="'.$st.'admin/zaloguj/" target="_top">
<label class="pl2">Email</label>
<input type="text" value="" name="login" id="ip1" class="travel-input inline-block relative block border p0 m0 mb1 mt1" placeholder="" required>
<label class="pl2">Hasło</label>
<input type="text" value="" name="haslo" id="ip1" class="travel-input inline-block relative block border p0 m0 mb1 mt1 pw" placeholder="" autocomplete="off" required>
<input type="submit"
    value="Zaloguj!"
    class="ampstart-btn caps">

<div submit-success>
<template type="amp-mustache">
<p class="center color-green">Logowanie udane. <a href="'.$st.'"><br />Przejdź na stronę główną!</a></p>
</template>
</div>
<div submit-error>
<template type="amp-mustache">
<p class="center">Logowanie nieudane!</p>
</template>
</div>
</form> 

...admin/zajoguj/ (from action-xhr) is php containing:

$output = array ('login' => $login, 'haslo' => $haslo); 
header("Content-type: application/json");
header("Access-Control-Allow-Credentials: true");
header("Access-Control-Allow-Origin: *.baza.testypilkarskie.pl");
header("AMP-Access-Control-Allow-Source-Origin: https://baza.testypilkarskie.pl");
header("Access-Control-Expose-Headers: AMP-Access-Control-Allow-Source-Origin");
echo json_encode($output);
die();

When I try this code, I dont get success form but i dont get error too, in Chrome console:

Failed to parse response JSON: SyntaxError: Unexpected token  in JSON at position 0

Code before headers (updating database and login in is working).

What is wrong?

Answer 1

I come back to this subject. There is answer how to fix extra dots in code (/ufeff):

https://github.com/contao/core/issues/6408