If you want to use the PayPal Payments Advanced with Magento, you must only use your PayPal Manager credentials please do not use your API Credentials.
This may be the prime reason behind getting Security Banner Header not valid 10002.
Following are the integration steps from Magento:
PayPal Advanced Integration with Magento
To begin with you need to setup Layout C in PayPal Manager PayPal Manager Login.
Please make sure that you are including your error URL, cancel URL, and Return URL. Magento is very peculiar about telling you about what URL you should be using.
After you are done with the setup of Layout C in PayPal Manager you need to use your Manager Credentials in Magento.
If you need them here is the information on setting up your Hosted Checkout Pages in PayPal Manager:
PayPal Hosted Checkout Pages
Following is the information on where to find your PayPal Manager credentials:
· Partner: It is your PayPal Partner ID. Most direct Merchants have this PayPal as the Partner
· Vendor: It is your PayPal user login name. Also known as Merchant Login
· User: It is the ID of an additional user set up on your PayPal account. It is same as Merchant login if you have not setup a separate user id in PayPal Manager.
Please use your API Signature Credentials from your PayPal account for the Express Checkout setup.