Paypal transaction error

I'm getting an error when I try to test the checkout. I'm using Paypal Express. I have debug mode on. I will post it in a private message.

The key part of this is: "errorSecurity header is not valid"

This means the username, password, or security key are incorrect.

That's odd. I just signed into Paypal using the same info that I have in the admin.

That shouldn't be your paypal login information... The API username and password is different.

Follow these steps:

Log in to your PayPal Business account.
Click the My Account tab.
Click the Profile tab. ...
Click Request API credentials under Account information.
Click Set up PayPal API credentials and permissions under Option 1.
Click Request API Credentials.
Click Request API signature.
Click Agree and Submit.

OH! Gotcha!

I got the API information and added it to the admin. Now I'm just getting a blank white screen when the order goes to Paypal.

I'll need a url to reproduce the problem and probably ftp access to debug. A blank page usually means an error on the page with error display turned off. Maybe a missing file or bad configuration information, but I'd have to debug to be sure.

Here is the site link: http://cosmicimaging.com/ The products are under Gallery.

I just tested and it seems to be working. At what point do you get the blank screen?