Solution: Magento Paypal Express & Standard Problem – The PayPal Gateway has rejected the request. Security header is not valid (# 10002: Security error).

It is well known that many German customers like to buy via PayPal because in an emergency they get their money back up to a certain amount.

Recently, our customers have been reporting this problem “Security header is not valid (# 10002: Security error).“. All PayPal orders are not carried out at all. The order is canceled. That means loss of sales.

This problem occurs both in very old Magento versions and in Magento 1.9.x.

Here is the solution to the problem that appears when buying with PayPal Express and recently with PayPal Standard.

Magento 1.9 and PayPal Express Checkout and in some cases PayPal Standard

The error message is: The PayPal gateway has rejected the request. Security header is not valid (# 10002: Security error).
This appears when the customer has selected PayPal Express Checkout in the ordering process and recently also PayPal Standard in a shop (Magento 1.9.4.x) that we programmed 12 years ago.

The reason for this is usually incorrect PayPal login details for PayPal Express Checkout. In this case you should check the API credentials (API user name, API password and signature) in the Magento admin area in the Expres Checkout area to see whether they are correct or whether the entries are missing? Then it should work again.
In the new case with PayPal Standard, the shop operator had to create an API and then enter it in the shop.