I have contacted PayPal but since it's weekend I'll be getting an update only on Monday. If you do see the error page, please refresh a couple of times. That should work. For any problems please don't hesitate to get in touch either here in the comments section or @justunfollow or email info [at] justunfollow (dot) com

Update: The issue was with one of the PayPal servers. PayPal has identified and taken steps to rectify the problem. You should not be seeing expired sessions while making payments now.