Why do I see "entity is deleted" on the thank you page after submitting a Donation?

Problem:

As a front-end website user submitting a Donation, I get a Thank You message but also an error message that says "entity is deleted" (screenshot: http://www.screencast.com/t/RohNUfjB1NON).

Cause:

Salesforce can’t find the campaign associated with your Donation because it has probably been deleted or does not exist.

Solution:

Login to the Soapbox Administrator and navigate to the Donation form in question. Click the Integration tab and then the Select link to match the Donation to an existing campaign.