That sort of thing means that your session data is getting dumped for some reason, which is usually the result of caching or CDN plugins/services on the WordPress site or on the web host / server directory.
It could also be from starting out on http:// before getting sent to PayPal, and then PayPal returning you to https:// (or vice-verse). If that happens then you would lose session data.
Please look into those things and get back to me. Thanks!