i wiped out my fresh install of drupal 5.7 and ubercart beta 4 and did a fresh install of drupal 5.7 and ubercart beta 2 on a lunar pages hosted accnt

i set up a test download product, checked out through paypal sandbox, completed payment and once again i am brought back to the shopping cart page which shows the file in the cart.

if i go to my accnt the file is there for download and i get the email notification.

i have workflow_ng set to:
Update order status on full payment
check order balance> Balance is less than or equal to $0.00.
action> order to completed

i see that their is a lot of discussion on this issue in previous posts and there may be some work around by modifying some code but if there is update on the uc_paypal module you may be back to where you were.