Checkout: no confirmation page, no payment forms

When you try to signup for the site the "processing" icon just spins and there's a "null" response from ajax call in the browser console. The user and the site are created if you use "Free site" option but there's never a thank you page. If you try to create a premium site, processing icon also spins and the payment form never shows up.
Please note that the site is on WPEngine and after full conflict test it looks like BuddyPress is causing it. With BP enabled this happens, with BP disabled Pro Sites work fine.