Okay, so I set up a mirror of my store.

I got it running fine, and did the upgrade. everything went smoothly.

I went to do a test (using a Gift Certificate to pay), and went through the checkout process.

I confirmed the order and got a blank page for the following URL:
index.php?main_page=checkout_process

At that point, I had not uninstalled and re-installed Paypal, so I did that, and then did the same with my shipping modules as well (UPS and USPS).

Still get the same problem. I also tried it, without a GV, and that goes to Paypal like it is supposed to. Even paying partially with GV sends the order to Paypal like it should. It is when the whole order is paid via a GV that this happens.

BTW, when this does happen, the order is placed/processed, but the shopping cart is not emptied (hope that helps)

I have the following things installed:
Gift Certificate Admin
Sales Report
User Tracker
EZ Populate
Glossy Black Theme

And I have done a few small language file tweaks (though i seriously doubt that that is the problem). I have also added in a couple of side boxes and a best seller page (linked to the sidebox) -- none of these small tweaks should have broken anything.

Any help or suggestions would be helpful.