As you can see in the screenshot of Firebug below the data coming in (from an Ajax-request to a PHP-page is a straightforward date as a string. And this string is successfully converted to a Date-object. But this always ends with a datePicker that states that the maxDate is an invalid Date. And therefore the maxDate does not do its trick.
Had this version of the site only on my local development machine so far so it took a bit of time to upload it. But you can see it now at www.picadevelopment.nl and there the problem comes back in the same way. Tried it with the latest versions of FireFox, Chrome and IE.