Upgrading from 5.19 to 5.21 broke several of my payment methods. The
problem happens in includes/classes/payment.php. To fix it, find the
OLD line and replace it with the NEW line.


OLD:
if (defined('MODULE_PAYMENT_INSTALLED') && tep_not_null(MODULE_PAYMENT_INSTALLED) && !$module == 'credit_covers') {


NEW:
if (defined('MODULE_PAYMENT_INSTALLED') && tep_not_null(MODULE_PAYMENT_INSTALLED) && $module != 'credit_covers') {


More...