If you are using PayPal Payments Advanced with Magento, you should be using your PayPal Manager credentials, and not your API Credentials.
This could be the reason you are getting Security Banner Header not valid 10002.
Here is the integration steps from Magento:
PayPal Advanced Integration with Magento
You need to setup Layout C in PayPal Manager PayPal Manager Login.
Make certain that you include your error URL, cancel URL, and Return URL. Magento is very good about telling you what URL you should be using.
After you have setup Layout C in PayPal Manager you need to use your Manager Credentials in Magento.
Just in case you need it, here is the information on setting up your Hosted Checkout Pages in PayPal Manager:
PayPal Hosted Checkout Pages
Here is the information on where to find your PayPal Manager credentials:
- Partner: Your PayPal Partner ID. Most direct Merchants have PayPal as the Partner
- Vendor: Your PayPal user login name. Also known as Merchant Login
- User: The ID of an additional user set up on your PayPal account. Same as Merchant login if you have not setup a separate user id in PayPal Manager.
Use your API Signature Credentials from your PayPal account for the Express Checkout setup.