MARKETPLACE: CHECKOUT not WORKING when I continue as guest

When someone places an order in the store and tries to checkout as guest (without registering on my website) their order placement isn't successful. They are redirected to paypal, they enter card details and then they're redirected to my homepage and the cart still has the item. But if someone has an account on my site everything works fine. Paypal guest is working fine too.

Can someone please help me? This is urgent and the WPMU "24x7" support is non-responsive