Hello! I am struggling to connect my Flutter frontend to my Phoenix backend. The trouble is with registering a user (in the database).

This is the router.ex entry for registering a user:

post "/register", UserRegistrationController, :create

(just the default one from mix.gen.auth)

I always get the error:
debug] ** (Plug.CSRFProtection.InvalidCSRFTokenError) invalid CSRF (Cross Site Request Forgery) token

Because my frontend and backend are completely segmented, I give the client their CSRF token through an api endpoint. I have confirmed that it is received by the client, and the flutter client sends the token in its http header.
Token generation by phoenix:

  def csrf_token(conn, _params) do
    token = Plug.CSRFProtection.get_csrf_token()
    json(conn, %{csrf_token: token})
  end

The flutter header with the token:

headers: {
'Content-Type': 'application/json',
 'X-CSRF-TOKEN': _csrfToken!,
 },

I would appreciate advice on what could be causing this error, I have been working on it for the past few days. Thank you.