When changing options on the My Subscriptions page, if you are using a version of jQuery lower than 1.10.x (which is included with the Subscribe Pro extension), the request will return a failed response because it is being sent as a GET request instead of a POST request. This is due to using the method parameter on the $.ajax() function. As per the jQuery API documentation the method parameter was added in version 1.9.0, and the correct parameter for earlier versions is type. The type parameter defaults to GET when it is omitted.
To ensure the Subscribe Pro extension behaves as expected, please use at least the version of jQuery provided with the extension package.