After having "PayPal IPN - Website Payments Standard" (herein: IPN) working very well for quite a while, I decided to switch to PayPal Express Checout (herein: PEC).

I followed the "Step 3: Setup API Access" instructions without any problems and proceeded to verify that it's working as intended.

I encountered the same exact problem described in this "Customer's PayPal account/credit card not debited" post.

I then followed the proposed solution of uninstalling disabled modules (I had two: "Credit Card - Offline Processing" and "Check/Money Order"). I was very hopeful that this would solve the problem.

Unfortunately, the problem only seems to have gotten worse:
  1. When customer clicks the "PayPal Express" button, she is properly directed her PayPal login page, but the order summary left pane is blank! (with IPN it shows the correct product description).
  2. Ignoring this problem, if the customer proceeds to login and confirming the purchase, she is returned to complete the purchase at my website (as expected) but then when she clicks the "Confirm your order" button, she is directed back to her PayPal account, instead of going directly to the download page! (all my products are digital/downloadables).

I know that PEC works very well for many others. How do I troubleshoot this problem?