marketpress / paypal issues

Running tests on my live website, using marketpress with globalcart and paypal express, all single store payments are going through perfectly but if i try and purchase a product from two stores it fails at the last confirmation page AFTER paypal.

I follow the checkout process successfully, get to paypal, thats successful, then returns to for final confirmation and this appears to fail, giving the customer this message: 'There was a problem finalizing your purchase with PayPal. Please go back and try again.' BUT when I go to my paypal I have received a payment and I get the emails about payment/new order etc. But the customer gets nothing - till they check their paypal account and discover the funds are missing.

Help - how can I fix this?