Same problem here, after paypal the shop site comes back with the message "Error occured:", but no error is listed, the payment is not registered in paypal and the order is not processe either, it still sits in the chart.
Testing here with prestashop 1.5.5 and the paypal sandbox.
Funny enough when I tried the live paypal, it all worked fine, but this is not very good for testing.
Have you been able to solve the problem?