Paypal Interruptus - Wordpress w/ CiviCRM 4.7.1
It probably means that paypal doesn't ping back properly your civi to let it know if it processed properly the card. Unfortunately, they are a lot of reasons to get that call not working, either because paypal try to IPN elsewhere, with a different format than expected or that something is wrong on how we process it (given it broke after an upgrade, the most likely option)

Can you check on the log if you have any error that might clarify what is the issue?