@Andrea Magni,
Thank you very much!
I solved the problem with your answer.
Moments before I see your answer, I discovered that the browser sends an OPTIONS before every request to know if the server accepts.
I in my ignorance had created an OPTIONS method on the server, returning an empty array and with CORS in the header, with the same url for each method (GET, PUT, ...) and had "solved" the problem.
I hope this post helps the beginners, as it helped this beginner.
Thank you!