I tried to book mid-April. After clicking the "Pay" button, the site displayed "An unknown error has occurred" and to please try again. I did a few times, gave up and called their customer service. The agent asked to send a screenshot, had no idea what the problem was or how to fix it, and offered to make the booking over the phone. I was worried she wouldn't get the same fare, but she did! Their system then sent me an email with a link to pay that did work. A bit clumsy, but really no big deal.