it should be the API username, api password, and api signature.
Since the pp_confirm page lods and does not go to the failure page, you have the correct values in the Get Payer Profile fro PayPal Express Checkout server behavior.
it is just in the Process transaction with PayPal Express checkout server behavior that it is set incorrectly.
dalso make sure that there are not 2 instances of the Process transaction with PayPal Express checkout server behavior this could also cause the problem.
make sure the api information in the Process transaction with PayPal Express checkout server behavior matches the api information in the Get Payer Profile fro PayPal Express Checkout server behavior