PayPal keeps redirecting back

On the page (you need to be logged in as a user) the PayPal button doesn't take you to a payment but instead redirects back. That's with "live" PayPal but I tested it also with my own PayPal sandbox credentials (so IPN was set properly) and it's the same. There's no debug.log created, I didn't find errors in the console. It's a clean setup.