Error: PayPal Check out with event registration. Fails to complete contribution and loses browser session
When using the PayPal Check out on events, end users have complained that they've gotten this error and returned them to an empty form:
Your browser session has expired and we are unable to complete your form submission. We have returned you to the initial step so you can complete and resubmit the form. If you experience continued difficulties, please contact us for assistance.
I've replicated the same issue using PayPal sandbox. PayPal Check out works on contribution forms.
The issue reported here sounds quite similar: https://civicrm.stackexchange.com/questions/29219/paypal-pro-button-not-working-returns-to-blank-event-form?rq=1