The SMS API displays the message "Incorrect username or password." Why is this?

It is probably because the password has (recently) changed. There are two ways to log in, and the passwords have to match. The two ways are:

  1. Through your Account; and
  2. Via the SMS API

Initially, the login information for both methods is identical but after changing either one of the passwords, the change does not automatically apply to the other login option. This is to ensure that existing applications do not have to be changed after a password change.

You can change your login password via the general settings page. As for the SMS API password, this can be changed at the API settings page.