For the sake of the developers, the issue is a JS error
Uncaught TypeError: Cannot read property 'price' of undefined
It seems to be the following line:
var regularPrice = response.data.price;
On line 441 of event-tickets/src/resources/js/tickets.js
The response to the ajax call seems to be
{"success":false,"message":"Cheatin' huh?"}
So I’m guessing a nonce isn’t validating somewhere
Tested on version 4.1