This is a common error that you might get if you are using Paypal Express Checkout, or Paypal Website Payments Pro. This means that there is something wrong with your login information that you are using to authenticate the connection with Paypal.
The resolution for this is most likely to remove your login information from the Paypal checkout settings within your BC control panel. Then re-enter that information. It is important to note that if you have copied & pasted this data that you will need to make sure that there no blank spaces before or after where you have highlighted to copy the text. We recommend that you first paste the information into a text editing program such as Notepad or TextEdit to ensure that there are no extra spaces.
Also, after verifying the login information, and/or re-entering it into your BC control panel, please make sure that you are not in sandbox mode with your PayPal account (both in the BC control panel settings, and within your PayPal settings.)