Appoinments+ confirmation problem on iPhone 6

Hello, my client has a problem with his site

When booking appointment on iPhone 6 the spinning loading animation just keeps spinnin after clicking confirmation button. This problem doesn't seem to appear on other devices, or atleast we haven't been able repeat it (we've tried other, iPhones, Android, desktop with multiple browsers).

Also the appointment gets booked, but you don't get notification email.

Any idea what might cause this?