I just upgraded and now have the same problem "on the checkout button the page simply refreshes". The buyer doesn't gets transferred to PayPal. Earlier it was working fine. My PayPal API is all set properly. May I request for a quick solution. Thanks.
The issue "on the checkout button the page simply refreshes" was solved by deactivating price tables. Have only one plan, hence it wasn't needed.
There is another issue of PayPal site giving certificate error. I guess the PayPal url in file class-paypal-api-helper.php should be https://cms.paypal.com/cgi-bin/webscr?cmd=_express-checkout instead of https://www.paypal.com/cgi-bin/webscr?cmd=_express-checkout.