Sandbox mode not working

When I attempt to complete a test pledge in Fundraising I get this error message:
There was a problem connecting with the paypal gateway. (CODE)520003 Authentication failed. API credentials are incorrect.

I am using Advanced Crowdfunding and have a business account with Paypal. I have not applied to the x.com site yet, so the Application ID is blank. Paypal mode is set to Sandbox.

Thanks