registration modal not working

As the title says, when reaching the payment page, one is forced to login or register. when the button is clicked, the modal pops up and the login form works find. But clicking the register tab results in nothing — no error, no error in the logs, nothing in the console…just a dead click.

Is there any way to disable the login requirement? Or, if not, fix this tab?

Thanks

The page I need help with: [log in to see the link]