Hi dowsed,
I had same problem with the paypal express checkout module, on a test site with XAMPP. It turned out it was timing out when doing the final communications with paypal I think. I altered timeout value in file plugins/wp-e-commerce/wpsc-merchants/paypal-express.merchant.php from 5 to 20 and that worked for me.
Whatever you are using as your payment module may be a similar solution.
To anyone encountering this with the paypal express module—
From memory the code appears to check for the timeout error but then doesn’t seem to finish handling the error completely. Hopefully justinsainton will see this and follow it up.
The timeout value was in 2 separate places in the file and I changed both.
Search the file for “timeout” and you will get both
