Hi Emilie,
I’m sorry to hear you’re running into this issue, but I’ll be happy to help you get it figured out.
Thanks for testing with a default theme, that is always a good first step! When you did this, did you also have all other plugins deactivated (so that only the Twenty Seventeen theme, The Events Calendar, Event Tickets, Event Tickets Plus, and WooCommerce were active)? If not, could you please give this a shot and see if you still run into this issue?
Could you also please send me a screenshot or preferably a link to your cart page?
Thanks,
Jennifer