i have the same problem and i have tried this with fresh installs (latest code) on 2 different servers with the same results.

using sandbox, i checkout, go through paypal but i get returned to the cart page. the file download does show up on my accounts page and i do get the confirmation with the link to the download.

i always get directed back to the cart page showing the item still in the cart.

i do not have a return address set in paypal.

i hired a ubercart specialist to try and figure this out and we wasted a lot of time but it looks like a bug maybe?