I may have responded to this one earlier. If you update the API credentials that you use for the checkout you will need to ensure that you get them updated in all places. This would include the checkout page, both confirm and pp_confirm, and any other page they may appear on.
The easiest way to do this is to do global find and replace, first for your API username, then again for the signature. If you do not get it updated in all areas it will not work properly. The other way to do this is to modify these server behaviors on these pages and update the values there, or recreate the checkout pages with the checkout wizard making sure to use the new credentials.