Appointments+/GCal Hangs on Submission

I asked this in the community forums, but haven't gotten an answer and I'm under pressure to get this resolved for a client.

I'm trying out Appointments+ for use with a client. I've installed it in a clean "sandbox" version of WP using the Twenty-Twelve theme - all other plugins disabled. However, when I choose a date and time to set an appointment, click the Google Calendar button and click to submit, I get the spinning "in progress" icon and it just hangs there. The appointment usually submits, but no GCal window is opened and I get no notification that my request has been submitted. And the spinner doesn't stop unless I reload the page.

Any idea what the issue might be?

You can try out the application at http://sandboxdd.navertise.net/schedule-an-appointment/. I've tried this on the latest versions of Chrome, Firefox and Safari.

Also, we're running on PHP 5.2.x. Is there any way to automatically get appointments posted to my client's GCal? I can't update the PHP at this time because it would affect dozens of client's sites.