I believe we found a bug in Magento and don't know how long this has been in play, but it's a showstopper for us. It can be reproduced under the following circumstances:
1. New customer adds something to shopping cart
2. Customer registers for new account
3. Customer pays with PayPal Express, is redirected to PayPal, clicks "Pay" to finalize payment and is returned back to home page instead of "Order Success" page.
4. Customer is unable to log in and view "My Account" page until after 12 hours.
We've been able to reproduce this behavior consistently on both our Development site and Production site. To eliminate any customization or extensions as being the cause, I installed a fresh 1.9.4.1 "stock" version of Magento and was able to reproduce this problem.
The problem is isolated to PayPal Express. We use PayFlow Pro to process credit card orders and that process is working fine for customers who follow the aforementioned steps. Also, if a customer has already created an account PRIOR to placing an order, the PayPal Express checkout process works.
