Authorize.NET only issue.
When updating the Credit Cards screen you get the error:
This is due to the Authorize.NET API returning a different Credit Card type for American Express than what we expected.
This was fixed in the 2016.07 release for new Credit Cards but you will need to run a query to fix it for existing Credit Cards already in the database:
UPDATE [@BOY_E0_CREDITCAEX] SET U_CardType = 'American Express' WHERE U_CardType = 'AmericanExpress'
This will make existing Credit Cards work and new once will work automatically when upgrading to 2016.07 or higher.