Question regarding error message fundraising and paypal

Hi --

I keep receiving this error message when I'm trying to process a pledge (using the Fundraiser plugin):

Your pledge of $5.00 is almost complete.
There was a problem connecting with the paypal gateway. (CODE)560029 The required X-PAYPAL-APPLICATION-ID header is missing from the HTTP request

...I have NO idea what this means... help?
Thank you!
Nadia