I am using Magento verion 1.4.0.1 and I had this issue. I had enabled Paypal Express Checkout payment method. When I choose this method on checkout, I was redirected to Paypal website but order data was not displayed there.

The main reason in my case was due to applied discount to the cart. I had created a shopping cart price rule and applied it to the cart. And, when I choose Paypal Express Checkout as payment method, the paypal website would show empty order data.

However, if I don’t apply discount to the cart, then everything is okay. While choosing Paypal Express, I am redirected to Paypal website and I could see order details there.

This article gives instructions on how to fix this. Please bear in mind this is for an older version of Magento, most likely 1.4.x