Appointment Request doesn't complete

When you click the book appointment button, it says
“Scheduling appointment, please wait.. “
but it never actually completes, it just keeps spinning. If you close the window then the appointment does actually go through and emails are sent.

http://cegielski.pixelpop.ca/?page_id=105

Console shows a 500 internal server error

https://wordpress.org/plugins/appointment-calendar/