If you have changed the username or password for an account, you will need to disable and re-enable downloading for the account in Banktivity. Please try the following steps:

• Click on the cloud icon beside the account name for the account. (or go to Account> Show Online Access Info)
• From the window that appears click the “Take Offline” button.
• Select this account from the left hand side of the iBank window by clicking on the account name (single-click).
• Go to Account> Set Up Online Access.
• Enter your account information in the windows that follow.

The above steps should work for accounts using either Direct Access or Direct Download (OFX).