I recently upgraded an installation of Magento from version 1.1.6 to 126.96.36.199 which lead to a couple issues with the payment system. The first issue was that I had to reset Base URL under Secure in the Configuration. The SSL settings seemed to have gotten lost during the upgrade.
This leads to the Payment transaction failed email I received when testing the checkout after my Magento upgrade. The Reason given in the email was “The merchant login ID or password is invalid or the account is inactive” and that is pretty much what’s happened after the Magento upgrade. Just as my Secure Base URL was wiped out during the upgrade so was my Authorize.Net API Login ID and Transaction Key.
So, to get payments working after my Magento upgrade I had to fetch my API Login ID and re-issue a Transaction Key at Authorize.Net. I then went into the Authorize.Net settings under the Magento Payment Methods Configurations screen and re-entered them.
After testing, this took care of the failed payment at Checkout after my Magento upgrade