IPS Connect INVALID_ACTION

I'm wondering if the documentation here is still valid.

No matter how much I try, this:

$check = file_get_contents( $masterUrl . '?' . http_build_query( array( 'act' => 'cookies', 'data' => json_encode( $_COOKIE ) ) ) );

always returns INVALID_ACTION (masterUrl is correct, cookie is set, user is logged in).

Any success with that?

Thanks