For 3 weeks already, I regularly check enrolling a student using PayPal sandbox.
When someone only quickly performs the necessary clicks in Paypal for the enrolment, the user is enrolled but redirected to the error page. However when someone takes his/her time to read the information provided by PayPal and only clicks every 3-10 seconds, this user does not see the error page and is enrolled correctly.
This second scenario is probably more accurate as most people read the screen when real money is involved (at least I do ). Moreover, I do not see any entries related to this error in the apache log files in non-sandbox environments.
So I can only conclude that the sandbox environment is working fine in Moodle 3.3, but that our test environments and/or the PayPal sandbox are not that fast as we would prefer.