Nieoczekiwany token w w JSON w pozycji 0
Unexpected token W in JSON at position 0
It's because a simple string (plain text) is returned as the response. The text is not a valid JSON. So when you try to do res.json(), it calls JSON.parse(data). Try and do it with the string you provided, and you will get the same error.
use res.text() instead of res.json()
aus ogola